The Evolution of Man

by Ernst Haeckel

SYNOPSIS OF THE CHIEF SECTIONS OF OUR STEM-HISTORY

First Stage: The Protists

Man’s ancestors are unicellular protozoa, originally unnucleated Monera like the Chromacea, structureless green particles of plasm; afterwards real nucleated cells (first plasmodomous Protophyta, like the Palmella; then plasmophagous Protozoa, like the Amœba).

Second Stage: The Blastæads

Man’s ancestors are round cœnobia or colonies of Protozoa; they consist of a close association of many homogeneous cells, and thus are individuals of the second order. They resemble the round cell-communities of the Magospheræ and Volvocina, equivalent to the ontogenetic blastula: hollow globules, the wall of which consists of a single layer of ciliated cells (blastoderm).

Third Stage: The Gastræads

Man’s ancestors are Gastræads, like the simplest of the actual Metazoa (Prophysema, Olynthus, Hydra, Pemmatodiscus). Their body consists merely of a primitive gut, the wall of which is made up of the two primary germinal layers.

Fourth Stage: The Platodes

Man’s ancestors have substantially the organisation of simple Platodes (at first like the cryptocœlic Platodaria, later like the rhabdocœlic Turbellaria). The leaf-shaped bilateral-symmetrical body has only one gut-opening, and develops the first trace of a nervous centre from the ectoderm in the middle line of the back (Figs. 239, 240).

Fifth Stage: The Vermalia

Man’s ancestors have substantially the organisation of unarticulated Vermalia, at first Gastrotricha (Ichthydina), afterwards Frontonia (Nemertina, Enteropneusta). Four secondary germinal layers develop, two middle layers arising between the limiting layers (cœloma). The dorsal ectoderm forms the vertical plate, acroganglion (Fig. 243).

Sixth Stage: The Prochordonia

Man’s ancestors have substantially the organisation of a simple unarticulated Chordonium (Copelata and Ascidia-larvæ). The unsegmented chorda develops between the dorsal medullary tube and the ventral gut-tube. The simple cœlom-pouches divide by a frontal septum into two on each side; the dorsal pouch (episomite) forms a muscle-plate; the ventral pouch (hyposomite) forms a gonad. Head-gut with gill-clefts.

Seventh Stage: The Acrania

Man’s ancestors are skull-less Vertebrates, like the Amphioxus. The body is a series of metamera, as several of the primitive segments are developed. The head contains in the ventral half the branchial gut, the trunk the hepatic gut. The medullary tube is still simple. No skull, jaws, or limbs.

Eighth Stage: The Cyclostoma

Man’s ancestors are jaw-less Craniotes (like the Myxinoida and Petromyzonta). The number of metamera increases. The fore-end of the medullary tube expands into a vesicle and forms the brain, which soon divides into five cerebral vesicles. In the sides of it appear the three higher sense-organs: nose, eyes, and auditory vesicles. No jaws, limbs, or floating bladder.

Ninth Stage: The Ichthyoda

Man’s ancestors are fish-like Craniotes: (1) Primitive fishes (Selachii); (2) plated fishes (Ganoida); (3) amphibian fishes (Dipneusta); (4) mailed amphibia (Stegocephala). The ancestors of this series develop two pairs of limbs: a pair of fore (breast-fins) and of hind (belly-fins) legs. The gill-arches are formed between the gill-clefts: the first pair form the maxillary arches (the upper and lower jaws). The floating bladder (lung) and pancreas grow out of the gut.

Tenth Stage: The Amniotes

Man’s ancestors are Amniotes or gill-less Vertebrates: (1) Primitive Amniotes (Proreptilia); (2) Sauromammals; (3) Primitive Mammals (Monotremes); (4) Marsupials; (5) Lemurs (Prosimiæ); (6) Western apes (Platyrrhinæ); (7) Eastern apes (Catarrhinæ): at first tailed Cynopitheca; then tail-less anthropoids; later speechless ape-men (Alali); finally speaking man. The ancestors of these Amniotes develop an amnion and allantois, and gradually assume the mammal, and finally the specifically human, form.

Chapter XXIV.
EVOLUTION OF THE NERVOUS SYSTEM

The previous chapters have taught us how the human body as a whole develops from the first simple rudiment, a single layer of cells. The whole human race owes its origin, like the individual man, to a simple cell. The unicellular stem-form of the race is reproduced daily in the unicellular embryonic stage of the individual. We have now to consider in detail the evolution of the various parts that make up the human frame. I must, naturally, confine myself to the most general and principal outlines; to make a special study of the evolution of each organ and tissue is both beyond the scope of this work, and probably beyond the anatomic capacity of most of my readers to appreciate. In tracing the evolution of the various organs we shall follow the method that has hitherto guided us, except that we shall now have to consider the ontogeny and phylogeny of the organs together. We have seen, in studying the evolution of the body as a whole, that phylogeny casts a light over the darker paths of ontogeny, and that we should be almost unable to find our way in it without the aid of the former. We shall have the same experience in the study of the organs in detail, and I shall be compelled to give simultaneously their ontogenetic and phylogenetic origin. The more we go into the details of organic development, and the more closely we follow the rise of the various parts, the more we see the inseparable connection of embryology and stem-history. The ontogeny of the organs can only be understood in the light of their phylogeny, just as we found of the embryology of the whole body. Each embryonic form is determined by a corresponding stem-form. This is true of details as well as of the whole.

We will consider first the animal and then the vegetal systems of organs of the body. The first group consists of the psychic and the motor apparatus. To the former belong the skin, the nervous system, and the sense-organs. The motor apparatus is composed of the passive and the active organs of movement (the skeleton and the muscles). The second or vegetal group consists of the nutritive and the reproductive apparatus. To the nutritive apparatus belong the alimentary canal with all its appendages, the vascular system, and the renal (kidney) system. The reproductive apparatus comprises the different organs of sex (embryonic glands, sexual ducts, and copulative organs).

As we know from previous chapters (XI–XIII), the animal systems of organs (the organs of sensation and presentation) develop for the most part out of the outer primary germ-layer, or the cutaneous (skin) layer. On the other hand, the vegetal systems of organs arise for the most part from the inner primary germ-layer, the visceral layer. It is true that this antithesis of the animal and vegetal spheres of the body in man and all the higher animals is by no means rigid; several parts of the animal apparatus (for instance, the greater part of the muscles) are formed from cells that come originally from the entoderm; and a great part of the vegetative apparatus (for instance, the mouth-cavity and the gonoducts) are composed of cells that come from the ectoderm.

In the more advanced animal body there is so much interlacing and displacement of the various parts that it is often very difficult to indicate the sources of them. But, broadly speaking, we may take it as a positive and important fact that in man and the higher animals the chief part of the animal organs comes from the ectoderm, and the greater part of the vegetative organs from the entoderm. It was for this reason that Carl Ernst von Baer called the one the animal and the other the vegetative layer (see p. 16).

The solid foundation of this important thesis is the gastrula, the most instructive embryonic form in the animal world, which we still find in the same shape in the most diverse classes of animals. This form points demonstrably to a common stem-form of all the Metazoa, the Gastræa; in this long-extinct stem-form the whole body consisted throughout life of the two primary germinal layers, as is now the case temporarily in the gastrula; in the Gastræa the simple cutaneous (skin) layer actually represented all the animal organs and functions, and the simple visceral (gut) layer all the vegetal organs and functions. This is the case with the modern Gastræads (Fig. 233); and it is also the case potentially with the gastrula.

We shall easily see that the gastræa theory is thus able to throw a good deal of light, both morphologically and physiologically, on some of the chief features of embryonic development, if we take up first the consideration of the chief element in the animal sphere, the psychic apparatus or sensorium and its evolution. This apparatus consists of two very different parts, which seem at first to have very little connection with each other—the outer skin, with all its hairs, nails, sweat-glands, etc., and the nervous system. The latter comprises the central nervous system (brain and spinal cord), the peripheral, cerebral, and spinal nerves, and the sense-organs. In the fully-formed vertebrate body these two chief elements of the sensorium lie far apart, the skin being external to, and the central nervous system in the very centre of, the body. The one is only connected with the other by a section of the peripheral nervous system and the sense-organs. Nevertheless, as we know from human embryology, the medullary tube is formed from the cutaneous layer. The organs that discharge the most advanced functions of the animal body—the organs of the soul, or of psychic life—develop from the external skin. This is a perfectly natural and necessary process. If we reflect on the historical evolution of the psychic and sensory functions, we are forced to conclude that the cells which accomplish them must originally have been located on the outer surface of the body. Only elementary organs in this superficial position could directly receive the influences of the environment. Afterwards, under the influence of natural selection, the cellular group in the skin which was specifically “sensitive” withdrew into the inner and more protected part of the body, and formed there the foundation of a central nervous organ. As a result of increased differentiation, the skin and the central nervous system became further and further separated, and in the end the two were only permanently connected by the afferent peripheral sensory nerves.

Fig. 284—The human skin in vertical section (from Ecker), highly magnified, a horny layer of the epidermis, b mucous layer of the epidermis, c papillæ of the corium, d blood-vessels of same, ef ducts of the sweat-glands (g), h fat-glands in the corium, i nerve, passing into a tactile corpuscle above.

The observations of the comparative anatomist are in complete accord with this view. He tells us that large numbers of the lower animals have no nervous system, though they exercise the functions of sensation and will like the higher animals. In the unicellular Protozoa, which do not form germinal layers, there is, of course, neither nervous system nor skin. But in the second division of the animal kingdom also, the Metazoa, there is at first no nervous system. Its functions are represented by the simple cell-layer of the ectoderm, which the lower Metazoa have inherited from the Gastræa (Fig. 30 e). We find this in the lowest Zoophytes—the Gastræads, Physemaria, and Sponges (Figs. 233–238). The lowest Cnidaria (the hydroid polyps) also are little superior to the Gastræads in structure. Their vegetative functions are accomplished by the simple visceral layer, and their animal functions by the simple cutaneous layer. In these cases the simple cell-layer of the ectoderm is at once skin, locomotive apparatus, and nervous system.

Fig. 285—Epidermic cells of a human embryo of two months. (From Kölliker.)

When we come to the higher Metazoa, in which the sensory functions and their organs are more advanced, we find a division of labour among the ectodermic cells. Groups of sensitive nerve cells separate from the ordinary epidermic cells; they retire into the more protected tissue of the mesodermic under-skin, and form special neural ganglia there. Even in the Platodes, especially the Turbellaria, we find an independent nervous system, which has separated from the outer skin. This is the “upper pharyngeal ganglion,” or acroganglion, situated above the gullet (Fig. 241 g).From this rudimentary structure has been developed the elaborate central nervous system of the higher animals. In some of the higher worms, such as the earth-worm, the first rudiment of the central nervous system (Fig. 74 n) is a local thickening of the skin-sense layer (hs), which afterwards separates altogether from the horny plate. In the earliest Platodes (Cryptocœla) and Vermalia (Gastrotricha) the acroganglion remains in the epidermis. But the medullary tube of the Vertebrates originates in the same way. Our embryology has taught us that this first structure of the central nervous system also develops originally from the outer germinal layer.

Let us now examine more closely the evolution of the human skin, with its various appendages, the hairs and glands. This external covering has, physiologically, a double and important part to play. It is, in the first place, the common integument that covers the whole surface of the body, and forms a protective envelope for the other organs. As such it also effects a certain exchange of matter between the body and the surrounding atmosphere (exhalation, perspiration). In the second place, it is the earliest and original sense organ, the common organ of feeling that experiences the sensation of the temperature of the environment and the pressure or resistance of bodies that come into contact.

The human skin (like that of all the higher animals) is composed of two layers, the outer and the inner or underlying skin. The outer skin or epidermis, consists of simple ectodermic cells, and contains no blood-vessels (Fig. 284 a, b). It develops from the outer germinal layer, or skin-sense layer. The underlying skin (corium or hypodermis) consists chiefly of connective tissue, contains numerous blood-vessels and nerves, and has a totally different origin. It comes from the outermost parietal stratum of the middle germinal layer, or the skin-fibre layer. The corium is much thicker than the epidermis. In its deeper strata (the subcutis) there are clusters of fat-cells (Fig. 284 h). Its uppermost stratum (the cutis proper, or the papillary stratum) forms, over almost the whole surface of the body, a number of conical microscopic papillæ (something like warts), which push into the overlying epidermis (c). These tactile or sensory particles contain the finest sensory organs of the skin, the touch corpuscles. Others contain merely end-loops of the blood-vessels that nourish the skin (c, d). The various parts of the corium arise by division of labour from the originally homogeneous cells of the cutis-plate, the outermost lamina of the mesodermic skin-fibre layer (Fig. 145 hpr, and Figs. 161, 162 cp).

In the same way, all the parts and appendages of the epidermis develop by differentiation from the homogeneous cells of this horny plate (Fig. 285). At an early stage the simple cellular layer of this horny plate divides into two. The inner and softer stratum (Fig. 284 b) is known as the mucous stratum, the outer and harder (a) as the horny (corneous) stratum. This horny layer is being constantly used up and rubbed away at the surface; new layers of cells grow up in their place out of the underlying mucous stratum. At first the epidermis is a simple covering of the surface of the body. Afterwards various appendages develop from it, some internally, others externally. The internal appendages are the cutaneous glands—sweat, fat, etc. The external appendages are the hairs and nails.

The cutaneous glands are originally merely solid cone-shaped growths of the epidermis, which sink into the underlying corium (Fig. 286 1). Afterwards a canal (2, 3) is formed inside them, either by the softening and dissolution of the central cells or by the secretion of fluid internally. Some of the glands, such as the sudoriferous, do not ramify (Fig. 284 efg). These glands, which secrete the perspiration, are very long, and have a spiral coil at the end, but they never ramify; so also the wax-glands of the ears. Most of the other cutaneous glands give out buds and ramify; thus, for instance, the lachrymal glands of the upper eye-lid that secrete tears (Fig. 286), and the sebaceous glands which secrete the fat in the skin and generally open into the hair-follicles. Sudoriferous and sebaceous glands are found only in mammals. But we find lachrymal glands in all the three classes of Amniotes—reptiles, birds, and mammals. They are wanting in the lower aquatic vertebrates.

Fig. 286—Rudimentary lachrymal glands from a human embryo of four months. (From Kölliker.) 1 earliest structure, in the shape of a simple solid cone, 2 and 3 more advanced structures, ramifying and hollowing out. a solid buds, e cellular coat of the hollow buds, f structure of the fibrous envelope, which afterwards forms the corium about the glands.

The mammary glands (Figs. 287, 288) are very remarkable; they are found in all mammals, and in these alone. They secrete the milk for the feeding of the new-born mammal. In spite of their unusual size these structures are nothing more than large sebaceous glands in the skin. The milk is formed by the liquefaction of the fatty milk-cells inside the branching mammary-gland tubes (Fig. 287 c), in the same way as the skin-grease or hair-fat, by the solution of fatty cells inside the sebaceous glands. The outlets of the mammary glands enlarge and form sac-like mammary ducts (b); these narrow again (a), and open in the teats or nipples of the breast by sixteen to twenty-four fine apertures. The first structure of this large and elaborate gland is a very simple cone in the epidermis, which penetrates into the corium and ramifies. In the new-born infant it consists of twelve to eighteen radiating lobes (Fig. 288). These gradually ramify, their ducts become hollow and larger, and rich masses of fat accumulate between the lobes. Thus is formed the prominent female breast (mamma), on the top of which rises the teat or nipple (mammilla). The latter is only developed later on, when the mammary gland is fully-formed; and this ontogenetic phenomenon is extremely interesting, because the earlier mammals (the stem-forms of the whole class) have no teats. In them the milk comes out through a flat portion of the ventral skin that is pierced like a sieve, as we still find in the lowest living mammals, the oviparous Monotremes of Australia. The young animal licks the milk from the mother instead of sucking it. In many of the lower mammals we find a number of milk-glands at different parts of the ventral surface. In the human female there is usually only one pair of glands, at the breast; and it is the same with the apes, bats, elephants, and several other mammals. Sometimes, however, we find two successive pairs of glands (or even more) in the human female. Some women have four or five pairs of breasts, like pigs and hedgehogs (Fig. 103). This polymastism points back to an older stem-form. We often find these accessory breasts in the male also (Fig. 103 D). Sometimes, moreover, the normal mammary glands are fully developed and can suckle in the male; but as a rule they are merely rudimentary organs without functions in the male. We have already (Chapter XI) dealt with this remarkable and interesting instance of atavism.

While the cutaneous glands are inner growths of the epidermis, the appendages which we call hairs and nails are external local growths in it. The nails (Ungues) which form important protective structures on the back of the most sensitive parts of our limbs, the tips of the fingers and toes, are horny growths of the epidermis, which we share with the apes. The lower mammals usually have claws instead of them; the ungulates, hoofs. The stem-form of the mammals certainly had claws; we find them in a rudimentary form even in the salamander. The horny claws are highly developed in most of the reptiles (Fig. 264), and the mammals have inherited them from the earliest representatives of this class, the stem-reptiles (Tocosauria). Like the hoofs (ungulæ) of the Ungulates, the nails of apes and men have been evolved from the claws of the older mammals. In the human embryo the first rudiment of the nails is found (between the horny and the mucous stratum of the epidermis) in the fourth month. But their edges do not penetrate through until the end of the sixth month.

Fig. 287—The female breast (mamma) in vertical section. c racemose glandular lobes, b enlarged milk-ducts, a narrower outlets, which open into the nipple. (From H. Meyer.)

The most interesting and important appendages of the epidermis are the hairs; on account of their peculiar composition and origin we must regard them as highly characteristic of the whole mammalian class. It is true that we also find hairs in many of the lower animals, such as insects and worms. But these hairs, like the hairs of plants, are thread-like appendages of the surface, and differ entirely from the hairs of the mammals in the details of their structure and development.

The embryology of the hairs is known in all its details, but there are two different views as to their phylogeny. On the older view the hairs of the mammals are equivalent or homologous to the feathers of the bird or the horny scales of the reptile. As we deduce all three classes of Amniotes from a common stem-group, we must assume that these Permian stem-reptiles had a complete scaly coat, inherited from their Carboniferous ancestors, the mailed amphibia (Stegocephala); the bony scales of their corium were covered with horny scales. In passing from aquatic to terrestrial life the horny scales were further developed, and the bony scales degenerated in most of the reptiles. As regards the bird’s feathers, it is certain that they are modifications of the horny scales of their reptilian ancestors. But it is otherwise with the hairs of the mammals. In their case the hypothesis has lately been advanced on the strength of very extensive research, especially by Friedrich Maurer, that they have been evolved from the cutaneous sense-organs of amphibian ancestors by modification of functions; the epidermic structure is very similar in both in its embryonic rudiments. This modern view, which had the support of the greatest expert on the vertebrates, Carl Gegenbaur, can be harmonised with the older theory to an extent, in the sense that both formations, scales and hairs, were very closely connected originally. Probably the conical budding of the skin-sense layer grew up under the protection of the horny scale, and became an organ of touch subsequently by the cornification of the hairs; many hairs are still sensory organs (tactile hairs on the muzzle and cheeks of many mammals: pubic hairs).

This middle position of the genetic connection of scales and hairs was advanced in my Systematic Phylogeny of the Vertebrates (p. 433). It is confirmed by the similar arrangement of the two cutaneous formations. As Maurer pointed out, the hairs, as well as the cutaneous sense-organs and the scales, are at first arranged in regular longitudinal series, and they afterwards break into alternate groups. In the embryo of a bear two inches long, which I owe to the kindness of Herr von Schmertzing (of Arva Varallia, Hungary), the back is covered with sixteen to twenty alternating longitudinal rows of scaly protuberances (Fig. 289). They are at the same time arranged in regular transverse rows, which converge at an acute angle from both sides towards the middle of the back. The tip of the scale-like wart is turned inwards. Between these larger hard scales (or groups of hairs) we find numbers of rudimentary smaller hairs.

The human embryo is, as a rule, entirely clothed with a thick coat of fine wool during the last three or four weeks of gestation. This embryonic woollen coat (Lanugo) generally disappears in part during the last weeks of fœtal life but in any case, as a rule, it is lost immediately after birth, and is replaced by the thinner coat of the permanent hair. These permanent hairs grow out of hair-follicles, which are formed from the root-sheaths of the disappearing wool-fibres. The embryonic wool-coat usually, in the case of the human embryo, covers the whole body, with the exception of the palms of the hands and soles of the feet. These parts are always bare, as in the case of apes and of most other mammals. Sometimes the wool-coat of the embryo has a striking effect, by its colour, on the later permanent hair-coat. Hence it happens occasionally, for instance, among our Indo-Germanic races, that children of blond parents seem—to the dismay of the latter—to be covered at birth with a dark brown or even a black woolly coat. Not until this has disappeared do we see the permanent blond hair which the child has inherited. Sometimes the darker coat remains for weeks, and even months, after birth. This remarkable woolly coat of the human embryo is a legacy from the apes, our ancient long-haired ancestors.

Fig. 288—Mammary gland of a new-born infant, a original central gland, b small and c large buds of same. (From Langer.)

It is not less noteworthy that many of the higher apes approach man in the thinness of the hair on various parts of the body. With most of the apes, especially the higher Catarrhines (or narrow-nosed apes), the face is mostly, or entirely, bare, or at least it has hair no longer or thicker than that of man. In their case, too, the back of the head is usually provided with a thicker growth of hair; this is lacking, however, in the case of the bald-headed chimpanzee (Anthropithecus calvus). The males of many species of apes have a considerable beard on the cheeks and chin; this sign of the masculine sex has been acquired by sexual selection. Many species of apes have a very thin covering of hair on the breast and the upper side of the limbs—much thinner than on the back or the under side of the limbs. On the other hand, we are often astonished to find tufts of hair on the shoulders, back, and extremities of members of our Indo-Germanic and of the Semitic races. Exceptional hair on the face, as on the whole body, is hereditary in certain families of hairy men. The quantity and the quality of the hair on head and chin are also conspicuously transmitted in families. These extraordinary variations in the total and partial hairy coat of the body, which are so noticeable, not only in comparing different races of men, but also in comparing different families of the same race, can only be explained on the assumption that in man the hairy coat is, on the whole, a rudimentary organ, a useless inheritance from the more thickly-coated apes. In this man resembles the elephant, rhinoceros, hippopotamus, whale, and other mammals of various orders, which have also, almost entirely or for the most part, lost their hairy coats by adaptation.

The particular process of adaptation by which man lost the growth of hair on most parts of his body, and retained or augmented it at some points, was most probably sexual selection. As Darwin luminously showed in his Descent of Man, sexual selection has been very active in this respect. As the male anthropoid apes chose the females with the least hair, and the females favoured the males with the finest growths on chin and head, the general coating of the body gradually degenerated, and the hair of the beard and head was more strongly developed. The growth of hair at other parts of the body (arm-pit, pubic region) was also probably due to sexual selection. Moreover, changes of climate, or habits, and other adaptations unknown to us, may have assisted the disappearance of the hairy coat.

Fig. 289—Embryo of a bear (Ursus arctos). A seen from ventral side, B from the left.

The fact that our coat of hair is inherited directly from the anthropoid apes is proved in an interesting way, according to Darwin, by the direction of the rudimentary hairs on our arms, which cannot be explained in any other way. Both on the upper and the lower part of the arm they point towards the elbow. Here they meet at an obtuse angle. This curious arrangement is found only in the anthropoid apes—gorilla, chimpanzee, orang, and several species of gibbons—besides man (Figs. 203, 207). In other species of gibbon the hairs are pointed towards the hand both in the upper and lower arm, as in the rest of the mammals. We can easily explain this remarkable peculiarity of the anthropoids and man on the theory that our common ancestors were accustomed (as the anthropoid apes are to-day) to place their hands over their heads, or across a branch above their heads, during rain. In this position, the fact that the hairs point downwards helps the rain to run off. Thus the direction of the hair on the lower part of our arm reminds us to-day of that useful custom of our anthropoid ancestors.

The nervous system in man and all the other Vertebrates is, when fully formed, an extremely complex apparatus, that we may compare, in anatomic structure and physiological function, with an extensive telegraphic system. The chief station of the system is the central marrow or central nervous system, the innumerable ganglionic cells or neurona (Fig. 9)of which are connected by branching processes with each other and with numbers of very fine conducting wires. The latter are the peripheral and ubiquitous nerve-fibres; with their terminal apparatus, the sense-organs, etc., they constitute the conducting marrow or peripheral nervous system. Some of them—the sensory nerve-fibres—conduct the impressions from the skin and other sense-organs to the central marrow; others—the motor nerve-fibres—convey the commands of the will to the muscles.

The central nervous system or central marrow (medulla centralis) is the real organ of psychic action in the narrower sense. However we conceive the intimate connection of this organ and its functions, it is certain that its characteristic actions, which we call sensation, will, and thought, are inseparably dependent on the normal development of the material organ in man and all the higher animals. We must, therefore, pay particular attention to the evolution of the latter. As it can give us most important information regarding the nature of the “soul,” it should be full of interest. If the central marrow develops in just the same way in the human embryo as in the embryo of the other mammals, the evolution of the human psychic organ from the central organ of the other mammals, and through them from the lower vertebrates, must be beyond question. No one can doubt the momentous bearing of these embryonic phenomena.

Fig. 290—Human embryo, three months old, from the dorsal side: brain and spinal cord exposed. (From Kölliker.) h cerebral hemispheres (fore brain), m corpora quadrigemina (middle brain), c cerebellum (hind brain): under the latter is the triangular medulla oblongata (after brain).
Fig. 291—Central marrow of a human embryo, four months old, from the back. (From Kölliker.) h large hemispheres, v quadrigemina, c cerebellum, mo medulla oblongata: underneath it the spinal cord.

In order to understand them fully we must first say a word or two of the general form and the anatomic composition of the mature human central marrow. Like the central nervous system of all the other Craniotes, it consists of two parts, the head-marrow or brain (medulla capitis or encephalon) and the spinal-marrow (medulla spinalis or notomyelon). The one is enclosed in the bony skull, the other in the bony vertebral column. Twelve pairs of cerebral nerves proceed from the brain, and thirty-one pairs of spinal nerves from the spinal cord, to the rest of the body (Fig. 171). On general anatomic investigation the spinal marrow is found to be a cylindrical cord, with a spindle-shaped bulb both in the region of the neck above (at the last cervical vertebra) and the region of the loins (at the first lumbar vertebra) below (Fig. 291). At the cervical bulb the strong nerves of the upper limbs, and at the lumbar bulb those of the lower limbs, proceed from the spinal cord. Above, the latter passes into the brain through the medulla oblongata (Fig. 291 mo). The spinal cord seems to be a thick mass of nervous matter, but it has a narrow canal at its axis, which passes into the further cerebral ventricles above, and is filled, like these, with a clear fluid.

The brain is a large nerve-mass, occupying the greater part of the skull, of most elaborate structure. On general examination it divides into two parts, the cerebrum and cerebellum. The cerebrum lies in front and above, and has the familiar characteristic convolutions and furrows on its surface (Figs. 292, 293). On the upper side it is divided by a deep longitudinal fissure into two halves, the cerebral hemispheres; these are connected by the corpus callosum. The large cerebrum is separated from the small cerebellum by a deep transverse furrow. The latter lies behind and below, and has also numbers of furrows, but much finer and more regular, with convolutions between, at its surface. The cerebellum also is divided by a longitudinal fissure into two halves, the “small hemispheres”; these are connected by a worm-shaped piece, the vermis cerebelli, above, and by the broad pons Varolii below (Fig. 292 VI).

Fig. 292—The human brain, seen from below. (From H. Meyer.) Above (in front) is the cerebrum with its extensive branching furrows; below (behind) the cerebellum with its narrow parallel furrows. The Roman numbers indicate the roots of the twelve pairs of cerebral nerves in a series towards the rear.

But comparative anatomy and ontogeny teach us that in man and all the other Craniotes the brain is at first composed, not of these two, but of three, and afterwards five, consecutive parts. These are found in just the same form—as five consecutive vesicles—in the embryo of all the Craniotes, from the Cyclostoma and fishes to man. But, however much they agree in their rudimentary condition, they differ considerably afterwards. In man and the higher mammals the first of these ventricles, the cerebrum, grows so much that in its mature condition it is by far the largest and heaviest part of the brain. To it belong not only the large hemispheres, but also the corpus callosum that unites them, the olfactory lobes, from which the olfactory nerves start, and most of the structures that are found at the roof and bottom of the large lateral ventricles inside the two hemispheres, such as the corpora striata. On the other hand, the optic thalami, which lie between the latter, belong to the second division, which develops from the “intermediate brain ”; to the same section belong the single third cerebral ventricle and the structures that are known as the corpora geniculata, the infundibulum, and the pineal gland. Behind these parts we find, between the cerebrum and cerebellum, a small ganglion composed of two prominences, which is called the corpus quadrigeminum on account of a superficial transverse fissure cutting across (Figs. 290 m and 291 v). Although this quadrigeminum is very insignificant in man and the higher mammals, it forms a special third section, greatly developed in the lower vertebrates, the “middle brain.” The fourth section is the “hind-brain” or little brain (cerebellum) in the narrower sense, with the single median part, the vermis, and the pair of lateral parts, the “small hemispheres” (Fig. 291 c). Finally, we have the fifth and last section, the medulla oblongata (Fig. 291 mo), which contains the single fourth cerebral cavity and the contiguous parts (pyramids, olivary bodies, corpora restiformia). The medulla oblongata passes straight into the medulla spinalis (spinal cord). The narrow central canal of the spinal cord continues above into the quadrangular fourth cerebral cavity of the medulla oblongata, the floor of which is the quadrangular depression. From here a narrow duct, called “the aqueduct of Sylvius,” passes through the corpus quadrigeminum to the third cerebral ventricle, which lies between the two optic thalami; and this in turn is connected with the pairs of lateral ventricles which lie to the right and left in the large hemispheres. Thus all the cavities of the central marrow are directly interconnected. All these parts of the brain have an infinitely complex structure in detail, but we cannot go into this. Although it is much more elaborate in man and the higher Vertebrates than in the lower classes, it develops in them all from the same rudimentary structure, the five simple cerebral vesicles of the embryonic brain.

But before we consider the development of the complicated structure of the brain from this simple series of vesicles, let us glance for a moment at the lower animals, which have no brain. Even in the skull-less vertebrate, the Amphioxus, we find no independent brain, as we have seen. The whole central marrow is merely a simple cylindrical cord which runs the length of the body, and ends equally simply at both extremities—a plain medullary tube. All that we can discover is a small vesicular bulb at the foremost part of the tube, a degenerate rudiment of a primitive brain. We meet the same simple medullary tube in the first structure of the ascidia larva, in the same characteristic position, above the chorda. On closer examination we find here also a small vesicular swelling at the fore end of the tube, the first trace of a differentiation of it into brain and spinal cord. It is probable that this differentiation was more advanced in the extinct Provertebrates, and the brain-bulb more pronounced (Figs. 98–102). The brain is phylogenetically older than the spinal cord, as the trunk was not developed until after the head. If we consider the undeniable affinity of the Ascidiæ to the Vermalia, and remember that we can trace all the Chordonia to lower Vermalia, it seems probable that the simple central marrow of the former is equivalent to the simple nervous ganglion, which lies above the gullet in the lower worms, and has long been known as the “upper pharyngeal ganglion” (ganglion pharyngeum superius); it would be better to call it the primitive or vertical brain (acroganglion).

Probably this upper pharyngeal ganglion of the lower worms is the structure from which the complex central marrow of the higher animals has been evolved. The medullary tube of the Chordonia has been formed by the lengthening of the vertical brain on the dorsal side. In all the other animals the central nervous system has been developed in a totally different way from the upper pharyngeal ganglion; in the Articulates, especially, a pharyngeal ring, with ventral marrow, has been added. The Molluscs also have a pharyngeal ring, but it is not found in the Vertebrates. In these the central marrow has been prolonged down the dorsal side; in the Articulates down the ventral side. This fact proves of itself that there is no direct relationship between the Vertebrates and the Articulates. The unfortunate attempts to derive the dorsal marrow of the former from the ventral marrow of the latter have totally failed (cf. p. 219).

Fig. 293—The human brain, seen from the left. (From H. Meyer.) The furrows of the cerebrum are indicated by thick, and those of the cerebellum by finer lines. Under the latter we can see the medulla oblongata. f1–f2 frontal convolutions, C central convolutions, S fissure of Sylvius, T temporal furrow, Pa parietal lobes, An angular gyrus, Po parieto-occipital fissure.

When we examine the embryology of the human nervous system, we must start from the important fact, which we have already seen, that the first structure of it in man and all the higher Vertebrates is the simple medullary tube, and that this separates from the outer germinal layer in the middle line of the sole-shaped embryonic shield. As the reader will remember, the straight medullary furrow first appears in the middle of the sandal-shaped embryonic shield. At each side of it the parallel borders curve over in the form of dorsal or medullary swellings. These bend together with their free borders, and thus form the closed medullary tube (Figs. 133–137). At first this tube lies directly underneath the horny plate; but it afterwards travels inwards, the upper edges of the provertebral plates growing together between the horny plate and the tube, joining above the latter, and forming a completely closed canal. As Gegenbaur very properly observes, “this gradual imbedding in the inner part of the body is a process acquired with the progressive differentiation and the higher potentiality that this secures; by this process the organ of greater value to the organism is buried within the frame.” (Cf. Figs. 143–146).

In the Cyclostoma—a stage above the Acrania—the fore end of the cylindrical medullary tube begins early to expand into a pear-shaped vesicle; this is the first outline of an independent brain. In this way the central marrow of the Vertebrates divides clearly into its two chief sections, brain and spinal cord. The simple vesicular form of the brain, which persists for some time in the Cyclostoma, is found also at first in all the higher Vertebrates (Fig. 153 hb). But in these it soon passes away, the one vesicle being divided into several successive parts by transverse constrictions. There are first two of these constrictions, dividing the brain into three consecutive vesicles (fore brain, middle brain, and hind brain, Fig. 154 v, m, h). Then the first and third are sub-divided by fresh constrictions, and thus we get five successive sections (Fig. 155).

Fig. 294–296—Central marrow of the human embryo from the seventh week, 4/5 inch long. (From Kölliker.) Fig. 294. The brain from above, v fore brain, z intermediate brain, m middle brain, h hind brain, n after brain. Fig. 2955. The brain with the uppermost part of the cord, from the left. Fig. 296. Back view of the whole embryo: brain and spinal cord exposed.

In all the Craniotes, from the Cyclostoma up to man, the same parts develop from these five original cerebral vesicles, though in very different ways. The first vesicle, the fore brain (Fig. 155 v), forms by far the largest part of the cerebrum—namely, the large hemispheres, the olfactory lobes, the corpora striata, the callosum, and the fornix. From the second vesicle, the intermediate brain (z), originate especially the optic thalami, the other parts that surround the third cerebral ventricle, and the infundibulum and pineal gland. The third vesicle, the middle brain (m), produces the corpora quadrigemina and the aqueduct of Sylvius. From the fourth vesicle, the hind brain (h), develops the greater part of the cerebellum—namely, the vermis and the two small hemispheres. Finally, the fifth vesicle, the after brain (n), forms the medulla oblongata, with the quadrangular pit (the floor of the fourth ventricle), the pyramids, olivary bodies, etc.

We must certainly regard it as a comparative-anatomical and ontogenetic fact of the greatest significance that in all the Craniotes, from the lowest Cyclostomes and fishes up to the apes and man, the brain develops in just the same way in the embryo. The first rudiment of it is always a simple vesicular enlargement of the fore end of the medullary tube. In every case, first three, then five, vesicles develop from this bulb, and the permanent brain with all its complex anatomic structures, of so great a variety in the various classes of Vertebrates, is formed from the five primitive vesicles. When we compare the mature brain of a fish, an amphibian, a reptile, a bird, and a mammal, it seems incredible that we can trace the various parts of these organs, that differ so much internally and externally, to common types. Yet all these different Craniote brains have started with the same rudimentary structure. To convince ourselves of this we have only to compare the corresponding stages of development of the embryos of these different animals.

This comparison is extremely instructive. If we extend it through the whole series of the Craniotes, we soon discover this interesting fact: In the Cyclostomes (the Myxinoida and Petromyzonta), which we have recognised as the lowest and earliest Craniotes, the whole brain remains throughout life at a very low stage, which is very brief and passing in the embryos of the higher Craniotes; they retain the five original sections of the brain unchanged. In the fishes we find an essential and considerable modification of the five vesicles; it is clearly the brain of the Selachii in the first place, and subsequently the brain of the Ganoids, from which the brain of the rest of the fishes on the one hand and of the Dipneusts and Amphibia, and through these of the higher Vertebrates, on the other hand, must be derived. In the fishes and Amphibia (Fig. 300) there is a preponderant development of the middle brain, and also the after brain, the first, second, and fourth sections remaining very primitive. It is just the reverse in the higher Vertebrates, in which the first and third sections, the cerebrum and cerebellum, are exceptionally developed; while the middle brain and after brain remain small. The corpora quadrigemina are mostly covered by the cerebrum, and the oblongata by the cerebellum. But we find a number of stages of development within the higher Vertebrates themselves. From the Amphibia upwards the brain (and with it the psychic life) develops in two different directions; one of these is followed by the reptiles and birds, and the other by the mammals. The development of the first section, the fore brain, is particularly characteristic of the mammals. It is only in them that the cerebrum becomes so large as to cover all the other parts of the brain (Figs. 293, 301–304).

Fig. 297—Head of a chick embryo (hatched fifty-eight hours), from the back. (From Mihalkovics.) vw anterior wall of the fore brain. vh its ventricle. au optic vesicles, mh middle brain, kh hind brain, nh after brain, hz heart (seen from below), vw vitelline veins, us primitive segment, rm spinal cord.

Fig. 298—Brain of three craniote embryos in vertical section. A of a shark (Heptarchus), B of a serpent (Coluber), C of a goat (Capra). a fore brain, b intermediate brain, c middle brain, d hind brain, e after brain, s primitive cleft. (From Gegenbaur.)
Fig. 299—Brain of a shark (Scyllium), back view. g fore-brain, h olfactory lobes, which send the large olfactory nerves to the nasal capsule (o), d intermediate brain, b middle brain; behind this the insignificant structure of the hind brain, a after brain. (From Gegenbaur.)

There are also notable variations in the relative position of the cerebral vesicles. In the lower Craniotes they lie originally almost in the same plane. When we examine the brain laterally, we can cut through all five vesicles with a straight line. But in the Amniotes there is a considerable curve in the brain along with the bending of the head and neck; the whole of the upper dorsal surface of the brain develops much more than the under ventral surface. This causes a curve, so that the parts come to lie as follows: The fore brain is right in front and below, the intermediate brain a little higher, and the middle brain highest of all; the hind brain lies a little lower, and the after brain lower still. We find this only in the Amniotes—the reptiles, birds, and mammals.

Thus, while the brain of the mammals agrees a good deal in general growth with that of the birds and reptiles, there are some striking differences between the two. In the Sauropsids (birds and reptiles) the middle brain and the middle part of the hind brain are well developed. In the mammals these parts do not grow, and the fore-brain develops so much that it overlies the other vesicles. As it continues to grow towards the rear, it at last covers the whole of the rest of the brain, and also encloses the middle parts from the sides (Figs. 301–303). This process is of great importance, because the fore brain is the organ of the higher psychic life, and in it those functions of the nerve-cells are discharged which we sum up in the word “soul.” The highest achievements of the animal body—the wonderful manifestations of consciousness and the complex molecular processes of thought—have their seat in the fore brain. We can remove the large hemispheres, piece by piece, from the mammal without killing it, and we then see how the higher functions of consciousness, thought, will, and sensation, are gradually destroyed, and in the end completely extinguished. If the animal is fed artificially, it may be kept alive for a long time, as the destruction of the psychic organs by no means involves the extinction of the faculties of digestion, respiration, circulation, urination—in a word, the vegetative functions. It is only conscious sensation, voluntary movement, thought, and the combination of various higher psychic functions that are affected.

Fig. 300—Brain and spinal cord of the frog. A from the dorsal, B from the ventral side. a olfactory lobes before the (b) fore brain, i infundibulum at the base of the intermediate brain, c middle brain, d hind brain, s quadrangular pit in the after brain, m spinal cord (very short in the frog), m′ roots of the spinal nerves, t terminal fibres of the spinal cord. (From Gegenbaur.)

The fore brain, the organ of these functions, only attains this high level of development in the more advanced Placentals, and thus we have the simple explanation of the intellectual superiority of the higher mammals. The soul of most of the lower Placentals is not much above that of the reptiles, but among the higher Placentals we find an uninterrupted gradation of mental power up to the apes and man. In harmony with this we find an astonishing variation in the degree of development of their fore brain, not only qualitatively, but also quantitatively. The mass and weight of the brain are much greater in modern mammals, and the differentiation of its various parts more important, than in their extinct Tertiary ancestors. This can be shown paleontologically in any particular order. The brains of the living ungulates are (relatively to the size of the body) four to six times (in the highest groups even eight times) as large as those of their earlier Tertiary ancestors, the well-preserved skulls of which enable us to determine the size and weight of the brain.

Fig. 301—Brain of an ox-embryo, two inches in length. (From Mihalkovics.) Left view; the lateral wall of the left hemisphere has been removed, st corpora striata, ml Monro-foramen, ag arterial plexus, ah Ammon’s horn, mh middle brain, kh cerebellum, dv roof of the fourth ventricle, bb pons Varolii, na medulla oblongata.

Fig. 302—Brain of a human embryo, twelve weeks old. (From Mihalkovics.) Seen from behind and above. ms mantle-furrow, mh corpora quadrigemina (middle brain), vs anterior medullary ala, kh cerebellum, vv fourth ventricle, na medulla oblongata.

In the lower mammals the surface of the cerebral hemispheres is quite smooth and level, as in the rabbit (Fig. 304). Moreover, the fore brain remains so small that it does not cover the middle brain. At a stage higher the middle brain is covered, but the hind brain remains free. Finally, in the apes and man, the latter also is covered by the fore brain. We can trace a similar gradual development in the fissures and convolutions that are found on the surface of the cerebrum of the higher mammals (Figs. 292, 293). If we compare different groups of mammals in regard to these fissures and convolutions, we find that their development proceeds step by step with the advance of mental life.

Fig. 303—Brain of a human embryo, twenty-four weeks old, halved in the median plane: right hemisphere seen from inside. (From Mihalkovics.) rn olfactory nerve, tr funnel of the intermediate brain, vc anterior commissure, ml Monro-foramen, gw fornix, ds transparent sheath, bl corpus callosum, br fissure at its border, hs occipital fissure, zh cuneus, sf occipital transverse fissure, zb pineal gland, mh corpora quadrigemina, kh cerebellum.

Of late years great attention has been paid to this special branch of cerebral anatomy, and very striking individual differences have been detected within the limits of the human race. In all human beings of special gifts and high intelligence the convolutions and fissures are much more developed than in the average man; and they are more developed in the latter than in idiots and others of low mental capacity. There is a similar gradation among the mammals in the internal structure of the fore brain. In particular the corpus callosum, that unites the two cerebral hemispheres, is only developed in the Placentals. Other structures—for instance, in the lateral ventricles—that seem at first to be peculiar to man, are also found in the higher apes, and these alone. It was long thought that man had certain distinctive organs in his cerebrum which were not found in any other animal. But careful examination has discovered that this is not the case, but that the characteristic features of the human brain are found in a rudimentary form in the lower apes, and are more or less fully developed in the higher apes. Huxley has convincingly shown, in his Man’s Place in Nature (1863), that the differences in the formation of the brain within the ape-group constitute a deeper gulf between the lower and higher apes than between the higher apes and man.

Fig. 304—Brain of the rabbit. A from the dorsal, B from the ventral side, lo olfactory lobes, I fore brain, h hypophysis at the base of the intermediate brain, III middle brain, IV hind brain, V after brain, 2 optic nerve, 3 oculo-motor nerve, 5–8 cerebral nerves. In A the roof of the right hemisphere (I) is removed, so that we can see the corpora striata in the lateral ventricle. (From Gegenbaur.)

The comparative anatomy and physiology of the brain of the higher and lower mammals are very instructive, and give important information in connection with the chief questions of psychology.

The central marrow (brain and spinal cord) develops from the medullary tube in man just as in all the other mammals, and the same applies to the conducting marrow or “peripheral nervous system.” It consists of the sensory nerves, which conduct centripetally the impressions from the skin and the sense-organs to the central marrow, and of the motor nerves, which convey centrifugally the movements of the will from the central marrow to the muscles. All these peripheral nerves grow out of the medullary tube (Fig. 171), and are, like it, products of the skin-sense layer.

The complete agreement in the structure and development of the psychic organs which we find between man and the highest mammals, and which can only be explained by their common origin, is of profound importance in the monistic psychology. This is only seen in its full light when we compare these morphological facts with the corresponding physiological phenomena, and remember that every psychic action requires the complete and normal condition of the correlative brain structure for its full and normal exercise. The very complex molecular movements inside the neural cells, which we describe comprehensively as “the life of the soul,” can no more exist in the vertebrate, and therefore in man, without their organs than the circulation without the heart and blood. And as the central marrow develops in man from the same medullary tube as that of the other vertebrates, and as man shares the characteristic structure of his cerebrum (the organ of thought) with the anthropoid apes, his psychic life also must have the same origin as theirs.

If we appreciate the full weight of these morphological and physiological facts, and put a proper phylogenetic interpretation on the observations of embryology, we see that the older idea of the personal immortality of the human soul is scientifically untenable. Death puts an end, in man as in any other vertebrate, to the physiological function of the cerebral neurona, the countless microscopic ganglionic cells, the collective activity of which is known as “the soul.” I have shown this fully in the eleventh chapter of my Riddle of the Universe.

Chapter XXV.
EVOLUTION OF THE SENSE-ORGANS

The sense-organs are indubitably among the most important and interesting parts of the human body; they are the organs by means of which we obtain our knowledge of objects in the surrounding world. Nihil est in intellectu quod non prius fuerit in sensu. They are the first sources of the life of the soul. There is no other part of the body in which we discover such elaborate anatomical structures, co-operating with a definite purpose; and there is no other organ in which the wonderful and purposive structure seems so clearly to compel us to admit a Creator and a preconceived plan. Hence we find special efforts made by dualists to draw our attention here to the “wisdom of the Creator” and the design visible in his works. As a matter of fact, you will discover, on mature reflection, that on this theory the Creator is at bottom only playing the part of a clever mechanic or watch-maker; all these familiar teleological ideas of Creator and creation are based, in the long run, on a similar childlike anthropomorphism.

However, we must grant that at the first glance the teleological theory seems to give the simplest and most satisfactory explanation of these purposive structures. If we merely examine the structure and functions of the most advanced sense-organs, it seems impossible to explain them without postulating a creative act. Yet evolution shows us quite clearly that this popular idea is totally wrong. With its assistance we discover that the purposive and remarkable sense-organs were developed, like all other organs, without any preconceived design—developed by the same mechanical process of natural selection, the same constant correlation of adaptation and heredity, by which the other purposive structures in the animal frame were slowly and gradually brought forth in the struggle for life.

Like most other Vertebrates, man has six sensory organs, which serve for eight different classes of sensations. The skin serves for sensations of pressure and temperature. This is the oldest, lowest, and vaguest of the sense-organs; it is distributed over the surface of the body. The other sensory activities are localised. The sexual sense is bound up with the skin of the external sexual organs, the sense of taste with the mucous lining of the mouth (tongue and palate), and the sense of smell with the mucous lining of the nasal cavity. For the two most advanced and most highly differentiated sensory functions there are special and very elaborate mechanical structures—the eye for the sense of sight, and the ear for the sense of hearing and space (equilibrium).

Comparative anatomy and physiology teach us that there are no differentiated sense-organs in the lower animals; all their sensations are received by the surface of the skin. The undifferentiated skin-layer or ectoderm of the Gastræa is the simple stratum of cells from which the differentiated sense-organs of all the Metazoa (including the Vertebrates) have been evolved. Starting from the assumption that necessarily only the superficial parts of the body, which are in direct touch with the outer world, could be concerned in the origin of sensations, we can see at once that the sense-organs also must have arisen there. This is really the case. The chief part of all the sense-organs originates from the skin-sense layer, partly directly from the horny plate, partly from the brain, the foremost part, of the medullary tube, after it has separated from the horny plate. If we compare the embryonic development of the various sense-organs, we see that they all make their appearance in the simplest conceivable form; the wonderful contrivances that make the higher sense-organs among the most remarkable and elaborate structures in the body develop only gradually. In the phylogenetic explanation of them comparative anatomy and ontogeny achieve their greatest triumphs. But at first all the sense-organs are merely parts of the skin in which sensory nerves expand. These nerves themselves were originally of a homogeneous character. The different functions or specific energies of the differentiated sense-nerves were only gradually developed by division of labour. At the same time, their simple terminal expansions in the skin were converted into extremely complex organs.

The great instructiveness of these historical facts in connection with the life of the soul is not difficult to see. The whole philosophy of the future will be transformed as soon as psychology takes cognisance of these genetic phenomena and makes them the basis of its speculations. When we examine impartially the manuals of psychology that have been published by the most distinguished speculative philosophers and are still widely distributed, we are astonished at the naivete with which the authors raise their airy metaphysical speculations, regardless of the momentous embryological facts that completely refute them. Yet the science of evolution, in conjunction with the great advance of the comparative anatomy and physiology of the sense-organs, provides the one sound empirical basis of a natural psychology.

Fig. 305—Head of a shark (Scyllium), from the ventral side. m mouth, o olfactory pits, r nasal groove, n nasal fold in natural position, n′ nasal fold drawn up. (The dots are openings of the mucous canals.) (From Gegenbaur.)

In respect of the terminal expansions of the sensory nerves, we can distribute the human sense-organs in three groups, which correspond to three stages of development. The first group comprises those organs the nerves of which spread out quite simply in the free surface of the skin itself (organs of the sense of pressure, warmth, and sex). In the second group the nerves spread out in the mucous coat of cavities which are at first depressions in or invaginations of the skin (organs of the sense of smell and taste). The third group is formed of the very elaborate organs, the nerves of which spread out in an internal vesicle, separated from the skin (organs of the sense of sight, hearing, and space).

There is little to be said of the development of the lower sense-organs. We have already considered (p. 268) the organ of touch and temperature in the skin. I need only add that in the corium of man and all the higher Vertebrates countless microscopic sense-organs develop, but the precise relation of these to the sensations of pressure or resistance, of warmth and cold, has not yet been explained. Organs of this kind, in or on which sensory cutaneous nerves terminate, are the “tactile corpuscles” (or the Pacinian corpuscles) and end-bulbs. We find similar corpuscles in the organs of the sexual sense, the male penis and the female clitoris; they are processes of the skin, the development of which we will consider later (together with the rest of the sexual parts, Chapter XXIX). The evolution of the organ of taste, the tongue and palate, will also be treated later, together with that of the alimentary canal to which these parts belong (Chapter XXVII). I will only point out for the present that the mucous coat of the tongue and palate, in which the gustatory nerve ends, originates from a part of the outer skin. As we have seen, the whole of the mouth-cavity is formed, not as a part of the gut-tube proper, but as a pit-like fold in the outer skin (p. 139). Its mucous lining is therefore formed, not from the visceral, but from the cutaneous layer, and the taste-cells at the surface of the tongue and palate are not products of the gut-fibre layer, but of the skin-sense layer.

Fig. 306 and 307—Head of a chick embryo, three days old: 2.306 front view, 2.307 from the right. n rudimentary nose (olfactory pits), l rudimentary eyes (optic pits), g rudimentary ear (auscultory pit), v fore brain, gl eye-cleft, o process of upper jaw, u process of lower jaw of the first gill-arch.

Fig. 308—Head of a chick embryo, four days old, from below. n nasal pit, o upper-jaw process of the first gill-arch, u lower-jaw process of same, k″ second gill-arch, sp choroid fissure of eye, s gullet.
Fig. 309 and 310—Heads of chick embryos: 309 from the end of the fourth, 310 from the beginning of the fifth week. Letters as in Fig. 308, except: in inner, an outer, nasal process, nf nasal furrow, st frontal process, m mouth. (From Kölliker.).

This applies also to the mucous lining of the olfactory organ, the nose. However, the development of this organ is much more interesting. Although the nose seems superficially to be simple and single, it really consists, in man and all other Gnathostomes, of two completely separated halves, the right and left cavities. They are divided by a vertical partition, so that the right nostril leads into the right cavity alone and the left nostril into the left cavity. They open internally (and separately) by the posterior nasal apertures into the pharynx, so that we can get direct into the gullet through the nasal passages without touching the mouth. This is the way the air usually passes in respiration; the mouth being closed, it goes through the nose into the gullet, and through the larynx and bronchial tubes into the lungs. The nasal cavities are separated from the mouth by the horizontal bony palate, to which is attached behind (as a dependent process) the soft palate with the uvula. In the upper and hinder parts of the nasal cavities the olfactory nerve, the first pair of cerebral nerves, expands in the mucous coat which clothes them. The terminal branches of it spread partly over the septum (partition), partly on the side walls of the internal cavities, to which are attached the turbinated bones. These bones are much more developed in many of the higher mammals than in man, but there are three of them in all mammals. The sensation of smell arises by the passage of a current of air containing odorous matter over the mucous lining of the cavities, and stimulating the olfactory cells of the nerve-endings.

Man has all the features which distinguish the olfactory organ of the mammals from that of the lower Vertebrates. In all essential points the human nose entirely resembles that of the Catarrhine apes, some of which have quite a human external nose (compare the face of the long-nosed apes). However, the first structure of the olfactory organ in the human embryo gives no indication of the future ample proportions of our catarrhine nose. It has the form in which we find it permanently in the fishes—a couple of simple depressions in the skin at the outer surface of the head. We find these blind olfactory pits in all the fishes; sometimes they lie near the eyes, sometimes more forward at the point of the muzzle, sometimes lower down, near the mouth (Fig. 249).

Fig. 311—Frontal section of the mouth and throat of a human embryo, neck half-inch long. “Invented” by Wilhelm His. The vertical section (in the frontal plane, from left to right) is so constructed that we see the nasal pits in the upper third of the figure and the eyes at the sides: in the middle third the primitive gullet with the gill-clefts (gill-arches in section); in the lower third the pectoral cavity with the bronchial tubes and the rudimentary lungs.

This first rudimentary structure of the double nose is the same in all the Gnathostomes; it has no connection with the primitive mouth. But even in a section of the fishes a connection of this kind begins to make its appearance, a furrow in the surface of the skin running from each side of the nasal pit to the nearest corner of the mouth. This furrow, the nasal groove or furrow (Fig. 305 r), is very important. In many of the sharks, such as the Scyllium, a special process of the frontal skin, the nasal fold or internal nasal process, is formed internally over the groove (n, n″). In contrast to this the outer edge of the furrow rises in an “external nasal process.” As the two processes meet and coalesce over the nasal groove in the Dipneusts and Amphibia, it is converted into a canal, the nasal canal. Henceforth we can penetrate from the external pits through the nasal canals direct into the mouth, which has been formed quite independently. In the Dipneusts and the lower Amphibia the internal aperture of the nasal canals lies in front (behind the lips); in the higher Amphibia it is right behind. Finally, in the three higher classes of Vertebrates the primary mouth-cavity is divided by the formation of the horizontal palate-roof into two distinct cavities—the upper (secondary) nasal cavity and the lower (secondary) mouth-cavity. The nasal cavity in turn is divided by the construction of the vertical septum into two halves—right and left.

Fig. 312—Diagrammatic section of the mouth-nose cavity. While the palate-plates (p) divide the original mouth-cavity into the lower secondary mouth (m) and the upper nasal cavity, the latter in turn is divided by the vertical partition (e) into two halves (n, n). (From Gegenbaur.)

Comparative anatomy shows us to-day, in the series of the double-nosed Vertebrates, from the fishes up to man, all the different stages in the development of the nose, which the advanced olfactory organ of the higher mammals has passed through at various periods in the course of its phylogeny. It first appears in the embryo of man and the higher Vertebrates, in which the double fish-nose persists throughout life. At an early stage, before there is any trace of the characteristic human face, a pair of small pits are formed in the head over the original mouth-cavity; these were first discovered by Baer, and rightly called the “olfactory pits” (Figs. 306 n, 307 n). These primitive nasal pits are quite separate from the rudimentary mouth, which also originates as a pit-like depression in the skin, in front of the blind fore end of the gut. Both the pair of nasal pits and the single mouth-pit (Fig. 310 m) are clothed with the horny plate. The original separation of the former from the latter is, however, presently abolished, a process forming above the mouth-pit—the “frontal process” (Fig. 309 st). Its outer edge rises to the right and left in the shape of two lateral processes; these are the inner nasal processes or folds (in). Opposite to these a parallel ridge is formed on either side between the eye and the nasal pit; these are the outer nasal processes (an). Thus between the inner and outer nasal processes a groove-like depression is formed on either side, which leads from the nasal pit towards the mouth-pit (m); this groove is, as the reader will guess, the same nasal furrow or groove that we have already seen in the shark (Fig. 305 r). As the parallel edges of the inner and outer nasal processes bend towards each other and join above the nasal groove, this is converted into a tube, the primitive nasal canal. Hence the nose of man and all the other Amniotes consists at this embryonic stage of a couple of narrow tubes, the nasal canals, which lead from the outer surface of the forehead into the rudimentary mouth. This transitory condition resembles that in which we find the nose permanently in the Dipneusts and Amphibia.

A cone-shaped structure, which grows from below towards the lower ends of the two nasal processes and joins with them, plays an important part in the conversion of the open nasal groove into the closed canal. This is the upper-jaw process (Figs. 306–310 o). Below the mouth-pit are the gill-arches, which are separated by the gill-clefts. The first of these gill-arches, and the most important for our purpose, which we may call the maxillary (jaw) arch, forms the skeleton of the jaws. Above at the basis a small process grows out of this first gill-arch; this is the upper-jaw process. The first gill-arch itself develops a cartilage at one of its inner sides, the “Meckel cartilage” (named after its discoverer), on the outer surface of which the lower jaw is formed (Figs. 306–310 u). The upper-jaw process forms the chief part of the skeleton of that jaw, the palate bone, and the pterygoid bone. On its outer side is afterwards formed the upper-jaw bone, in the narrower sense, while the middle part of the skeleton of the upper jaw, the intermaxillary, develops from the foremost part of the frontal process.

The two upper-jaw processes are of great importance in the further development of the face. From them is formed, growing into the primitive mouth-cavity, the important horizontal partition (the palate) that divides the former into two distinct cavities. The upper cavity, into which the nasal canals open, now develops into the nasal cavity, the air-passage and the organ of smell. The lower cavity forms the permanent secondary mouth (Fig. 312 m), the food-passage and the organ of taste. Both the upper and lower cavities open behind into the gullet (pharynx). The hard palate that separates them is formed by the joining of two lateral halves, the horizontal plates of the two upper-jaw processes, or the palate-plates (p). When these do not, sometimes, completely join in the middle, a longitudinal cleft remains, through which we can penetrate from the mouth straight into the nasal cavity. This is the malformation known as “wolf’s throat.” “Hare-lip” is the lesser form of the same defect. At the same time as the horizontal partition of the hard palate a vertical partition is formed by which the single nasal cavity is divided into two sections—a right and left half (Fig. 312 n, n).

Figs. 313 and 314—Upper part of the body of a human embryo, two-thirds of an inch long, of the sixth week; Fig. 313 from the left, Fig. 314 from the front. The origin of the nose and the upper lip from two lateral and originally separate halves can be clearly seen. Nose and upper lip are large in proportion to the rest of the face, and especially to the lower lip. (From Kollmann.)

The double nose has now acquired the characteristic form that man shares with the other mammals. Its further development is easy to follow; it consists of the formation of the inner and outer processes of the walls of the two cavities. The external nose is not formed until long after all these essential parts of the internal organ of smell. The first traces of it in the human embryo are found about the middle of the second month (Figs. 313–316). As can be seen in any human embryo during the first month, there is at first no trace of the external nose. It only develops afterwards from the foremost nasal part of the primitive skull, growing forwards from behind. The characteristic human nose is formed very late. Much stress is at times laid on this organ as an exclusive privilege of man. But there are apes that have similar noses, such as the long-nosed ape.

The evolution of the eye is not less interesting and instructive than that of the nose. Although this noblest of the sensory organs is one of the most elaborate and purposive on account of its optic perfection and remarkable structure, it nevertheless develops, without preconceived design, from a simple process of the outer germinal layer. The fully-formed human eye is a round capsule, the eye-ball (Fig. 317). This lies in the bony cavity of the skull, surrounded by protective fat and motor muscles. The greater part of it is taken up with a semi-fluid, transparent gelatinous substance, the corpus vitreum. The crystalline lens is fitted into the anterior surface of the ball (Fig. 317 l). It is a lenticular, bi-convex, transparent body, the most important of the refractive media in the eye. Of this group we have, besides the corpus vitreum and the lens, the watery fluid (humor aqueus) that is found in front of the lens (at the letter m in Fig. 317). These three transparent refractive media, by which the rays of light that enter the eye are broken up and re-focussed, are enclosed in a solid round capsule, composed of several different coats, something like the concentric layers of an onion. The outermost and thickest of these envelopes is the white sclerotic coat of the eye. It consists of tough white connective tissue. In front of the lens a circular, strongly-curved, transparent plate is fitted into the sclerotic, like the glass of a watch—the cornea (b). At its outer surface the cornea is covered with a very thin layer of the epidermis; this is known as the conjunctiva. It goes from the cornea over the inner surface of the eye-lids, the upper and lower folds which we draw over the eye in closing it. At the inner corner of the eye we have a rudimentary organ in the shape of the relic of a third (inner) eye-lid, which is greatly developed, as “nictitating (winking) membrane,” in the lower Vertebrates (p. 5). Underneath the upper eye-lid are the lachrymal glands, the product of which, the lachrymal fluid, keeps the outer surface of the eye smooth and clean.

Fig. 315—Face of a human embryo, seven weeks old. (From Kollmann.) Joining of the nasal processes (e outer, i inner) with the upper-jaw process (o), n nasal wall, a ear-opening.

Immediately under the sclerotic we find a very delicate, dark-red membrane, very rich in blood-vessels—the choroid coat—and inside this the retina (o), the expansion of the optic nerve (i). The latter is the second cerebral nerve. It proceeds from the optic thalami (the second cerebral vesicle) to the eye; penetrates its outer envelopes, and then spreads out like a net between the choroid and the corpus vitreum. Between the retina and the choroid there is a very delicate membrane, which is usually (but wrongly) associated with the latter. This is the black pigment-membrane (n). It consists of a single stratum of graceful, hexagonal, regularly-joined cells, full of granules of black colouring matter. This pigment membrane clothes, not only the inner surface of the choroid proper, but also the hind surface of its anterior muscular continuation, which covers the edge of the lens in front as a circular membrane, and arrests the rays of light at the sides. This is the well-known iris of the eye (h), coloured differently in different individuals (blue, grey, brown, etc.); it forms the anterior border of the choroid. The circular opening that is left in the middle is the pupil, through which the rays of light penetrate into the eye. At the point where the iris leaves the anterior border of the choroid proper the latter is very thick, and forms a delicate crown of folds (g), which surrounds the edge of the lens with about seventy large and many smaller rays (corona ciliaris.)

Fig. 316—Face of a human embryo, eight weeks old. (From Ecker.)

At a very early stage a couple of pear-shaped vesicles develop from the foremost part of the first cerebral vesicle in the embryo of man and the other Craniotes (Figs. 155 a, 297 au). These growths are the primary optic vesicles. They are at first directed outwards and forwards, but presently grow downward, so that, after the complete separation of the five cerebral vesicles, they lie at the base of the intermediate brain. The inner cavities of these pear-shaped vesicles, which soon attain a considerable size, are openly connected with the ventricle of the intermediate brain by their hollow stems. They are covered externally by the epidermis.

At the point where this comes into direct contact with the most curved part of the primary optic vesicle there is a thickening (l) and also a depression (o) of the horny plate (Fig. 318, I). This pit, which we may call the lens-pit, is converted into a closed sac, the thick- walled lens-vesicle (2, l), the thick edges of the pit joining together above it. In the same way in which the medullary tube separates from the outer germinal layer, we now see this lens-sac sever itself entirely from the horny plate (h), its source of origin. The hollow of the sac is afterwards filled with the cells of its thick walls, and thus we get the solid crystalline lens. This is, therefore, a purely epidermic structure. Together with the lens the small underlying piece of corium-plate also separates from the skin.

As the lens separates from the corneous plate and grows inwards, it necessarily hollows out the contiguous primary optic vesicle (Fig. 318, 1–3). This is done in just the same way as the invagination of the blastula, which gives rise to the gastrula in the amphioxus (Fig. 38 C–F). In both cases the hollowing of the closed vesicle on one side goes so far that at last the inner, folded part touches the outer, not folded part, and the cavity disappears. As in the gastrula the first part is converted into the entoderm and the latter into the ectoderm, so in the invagination of the primary optic vesicle the retina (r) is formed from the first (inner) part, and the black pigment membrane (u) from the latter (outer, non-invaginated) part. The hollow stem of the primary optic vesicle is converted into the optic nerve. The lens (l), which has so important a part in this process, lies at first directly on the invaginated part, or the retina (r). But they soon separate, a new structure, the corpus vitreum (gl), growing between them. While the lenticular sac is being detached and is causing the invagination of the primary optic vesicle, another invagination is taking place from below; this proceeds from the superficial part of the skin-fibre layer—the corium of the head. Behind and under the lens a last-shaped process rises from the cutis-plate (Fig. 319 g), hollows out the cup-shaped optic vesicle from below, and presses between the lens (l) and the retina (i). In this way the optic vesicle acquires the form of a hood.

Fig. 317—The human eye in section. a sclerotic coat, b cornea, c conjunctiva, d circular veins of the iris, e choroid coat, f ciliary muscle, g corona ciliaris, h iris, i optic nerve, k anterior border of the retina, l crystalline lens, m inner covering of the cornea (aqueous membrane), n pigment membrane, o retina, p Petit’s canal, q yellow spot of the retina. (From Helmholtz.)

Finally, a complete fibrous envelope, the fibrous capsule of the eye-ball, is formed about the secondary optic vesicle and its stem (the secondary optic nerve). It originates from the part of the head-plates which immediately encloses the eye. This fibrous envelope takes the form of a closed round vesicle, surrounding the whole of the ball and pushing between the lens and the horny plate at its outer side. The round wall of the capsule soon divides into two different membranes by surface-cleavage. The inner membrane becomes the choroid or vascular coat, and in front the ciliary corona and iris. The outer membrane is converted into the white protective or sclerotic coat—in front, the transparent cornea. The eye is now formed in all its essential parts. The further development—the complicated differentiation and composition of the various parts—is a matter of detail.

The chief point in this remarkable evolution of the eye is the circumstance that the optic nerve, the retina, and the pigment membrane originate really from a part of the brain—an outgrowth of the intermediate brain—while the lens, the chief refractive body, develops from the outer skin. From the skin—the horny plate—also arises the delicate conjunctiva, which afterwards covers the outer surface of the eyeball. The lachrymal glands are ramified growths from the conjunctiva (Fig. 286). All these important parts of the eye are products of the outer germinal layer. The remaining parts—the corpus vitreum (with the vascular capsule of the lens), the choroid (with the iris), and the sclerotic (with the cornea)—are formed from the middle germinal layer.

Fig. 318—Eye of the chick embryo in longitudinal section (1. from an embryo sixty-five hours old; 2. from a somewhat older embryo; 3. from an embryo four days old). h horny plate, o lens-pit, l lens (in 1. still part of the epidermis, in 2. and 3. separated from it), x thickening of the horny plate at the point where the lens has severed itself, gl corpus vitreum, r retina, u pigment membrane. (From Remak.)

The outer protection of the eye, the eye-lids, are merely folds of the skin, which are formed in the third month of human embryonic life. In the fourth month the upper eye-lid reaches the lower, and the eye remains covered with them until birth. As a rule, they open wide shortly before birth (sometimes only after birth). Our craniote ancestors had a third eye-lid, the nictitating membrane, which was drawn over the eye from its inner angle. It is still found in many of the Selachii and Amniotes. In the apes and man it has degenerated, and there is now only a small relic of it at the inner corner of the eye, the semi-lunar fold, a useless rudimentary organ (cf. p. 32). The apes and man have also lost the Harderian gland that opened under the nictitating membrane; we find this in the rest of the mammals, and the birds, reptiles, and amphibia.

The peculiar embryonic development of the vertebrate eye does not enable us to draw any definite conclusions as to its obscure phylogeny; it is clearly cenogenetic to a great extent, or obscured by the reduction and curtailment of its original features. It is probable that many of the earlier stages of its phylogeny have disappeared without leaving a trace. It can only be said positively that the peculiar ontogeny of the complicated optic apparatus in man follows just the same laws as in all the other Vertebrates. Their eye is a part of the fore brain, which has grown forward towards the skin, not an original cutaneous sense-organ, as in the Invertebrates.

Fig. 319—Horizontal transverse section of the eye of a human embryo, four weeks old. (From Kölliker.) t lens (the dark wall of which is as thick as the diameter of the central cavity), g corpus vitreum (connected by a stem, g, with the corium), v vascular loop (pressing behind the lens inside the corpus vitreum by means of this stem g), i retina (inner thicker, invaginated layer of the primary optic vesicle), a pigment membrane (outer, thin, non-invaginated layer of same), h space between retina and pigment membrane (remainder of the cavity of the primary optic vesicle).

The vertebrate ear resembles the eye and nose in many important respects, but is different in others, in its development. The auscultory organ in the fully-developed man is like that of the other mammals, and especially the apes, in the main features. As in them, it consists of two chief parts—an apparatus for conducting sound (external and middle ear) and an apparatus for the sensation of sound (internal ear). The external ear opens in the shell at the side of the head (Fig. 320 a). From this point the external passage (b), about an inch in length, leads into the head. The inner end of it is closed by the tympanum, a vertical, but not quite upright, thin membrane of an oval shape (c). This tympanum separates the external passage from the tympanic cavity (d). This is a small cavity, filled with air, in the temporal bone; it is connected with the mouth by a special tube. This tube is rather longer, but much narrower, than the outer passage, leads inwards obliquely from the anterior wall of the tympanic cavity, and opens in the throat below, behind the nasal openings. It is called the Eustachian tube (e); it serves to equalise the pressure of the air within the tympanic cavity and the outer atmosphere that enters by the external passage. Both the Eustachian tube and the tympanic cavity are lined with a thin mucous coat, which is a direct continuation of the mucous lining of the throat. Inside the tympanic cavity there are three small bones which are known (from their shape) as the hammer, anvil, and stirrup (Fig. 320, f, g, h). The hammer (f) is the outermost, next to the tympanum. The anvil (g) fits between the other two, above and inside the hammer. The stirrup (h) lies inside the anvil, and touches with its base the outer wall of the internal ear, or auscultory vesicle. All these parts of the external and middle ear belong to the apparatus for conducting sound. Their chief task is to convey the waves of sound through the thick wall of the head to the inner-lying auscultory vesicle. They are not found at all in the fishes. In these the waves of sound are conveyed directly by the wall of the head to the auscultory vesicle.

Fig. 320—The human ear (left ear, seen from the front), a shell of ear, b external passage, c tympanum, d tympanic cavity, e Eustachian tube, f, g, h the three bones of the ear (f hammer, g anvil, h stirrup), i utricle, k the three semi-circular canals, l the sacculus, m cochlea, n auscultory nerve.

Fig. 321—The bony labyrinth of the human ear (left side). a vestibulum, b cochlea, c upper canal, d posterior canal, e outer canal, f oval fenestra, g round fenestra. (From Meyer.)

The internal apparatus for the sensation of sound, which receives the waves of sound from the conducting apparatus, consists in man and all other mammals of a closed auscultory vesicle filled with fluid and an auditory nerve, the ends of which expand over the wall of this vesicle. The vibrations of the sound-waves are conveyed by these media to the nerve-endings. In the labyrinthic water that fills the auscultory vesicle there are small stones at the points of entry of the acoustic nerves, which are composed of groups of microscopic calcareous crystals (otoliths). The auscultory organ of most of the Invertebrates has substantially the same composition. It usually consists of a closed vesicle, filled with fluid, and containing otoliths, with the acoustic nerve expanding on its wall. But, while the auditory vesicle is usually of a simple round or oval shape in the Invertebrates, it has in the Vertebrates a special and curious structure, the labyrinth. This thin-membraned labyrinth is enclosed in a bony capsule of the same shape, the osseous labyrinth (Fig. 321), and this lies in the middle of the petrous bone of the skull. The labyrinth is divided into two vesicles in all the Gnathostomes. The larger one is called the utriculus, and has three arched appendages, called the “semi-circular canals” (c, d, e). The smaller vesicle is called the sacculus, and is connected with a peculiar appendage, with (in man and the higher mammals) a spiral form something like a snail’s shell, and therefore called the cochlea (= snail, b). On the thin wall of this delicate labyrinth the acoustic nerve, which comes from the after-brain, spreads out in most elaborate fashion. It divides into two main branches—a cochlear nerve (for the cochlea) and a vestibular nerve (for the rest of the labyrinth). The former seems to have more to do with the quality, the latter with the quantity, of the acoustic sensations. Through the cochlear nerves we learn the height and timbre, through the vestibular nerves the intensity, of tones.

The first structure of this highly elaborate organ is very simple in the embryo of man and all the other Craniotes; it is a pit-like depression in the skin. At the back part of the head at both sides, near the after brain, a small thickening of the horny plate is formed at the upper end of the second gill-cleft (Fig. 322 A fl). This sinks into a sort of pit, and severs from the epidermis, just as the lens of the eye does. In this way is formed at each side, directly under the horny plate of the back part of the head, a small vesicle filled with fluid, the primitive auscultory vesicle, or the primary labyrinth. As it separates from its source, the horny plate, and presses inwards and backwards into the skull, it changes from round to pear-shaped (Figs. 322 B lv, 323 o). The outer part of it is lengthened into a thin stem, which at first still opens outwards by a narrow canal. This is the labyrinthic appendage (Fig. 322 lr). In the lower Vertebrates it develops into a special cavity filled with calcareous crystals, which remains open permanently in some of the primitive fishes, and opens outwards in the upper part of the skull. But in the mammals the labyrinthic appendage degenerates. In these it has only a phylogenetic interest as a rudimentary organ, with no actual physiological significance. The useless relic of it passes through the wall of the petrous bone in the shape of a narrow canal, and is called the vestibular aqueduct.

Fig. 322—Development of the auscultory labyrinth of the chick, in five successive stages (A–E). (Vertical transverse sections of the skull.) fl auscultory pits, lv auscultory vesicles, lr labyrinthic appendage, c rudimentary cochlea, csp posterior canal, cse external canal, jv jugular vein. (From Reissner.)

It is only the inner and lower bulbous part of the separated auscultory vesicle that develops into the highly complex and differentiated structure that is afterwards known as the secondary labyrinth. This vesicle divides at an early stage into an upper and larger and a lower and smaller section. From the one we get the utriculus with the semi-circular canals; from the other the sacculus and the cochlea (Fig. 320 c). The canals are formed in the shape of simple pouch-like involutions of the utricle (cse and csp). The edges join together in the middle part of each fold, and separate from the utricle, the two ends remaining in open connection with its cavity. All the Gnathostomes have these three canals like man, whereas among the Cyclostomes the lampreys have only two and the hag-fishes only one. The very complex structure of the cochlea, one of the most elaborate and wonderful outcomes of adaptation in the mammal body, develops originally in very simple fashion as a flask-like projection from the sacculus. As Hasse and Retzius have pointed out, we find the successive ontogenetic stages of its growth represented permanently in the series of the higher Vertebrates. The cochlea is wanting even in the Monotremes, and is restricted to the rest of the mammals and man.

The auditory nerve, or eighth cerebral nerve, expands with one branch in the cochlea, and with the other in the remaining parts of the labyrinth. This nerve is, as Gegenbaur has shown, the sensory dorsal branch of a cerebro-spinal nerve, the motor ventral branch of which acts for the muscles of the face (nervus facialis). It has therefore originated phylogenetically from an ordinary cutaneous nerve, and so is of quite different origin from the optic and olfactory nerves, which both represent direct outgrowths of the brain. In this respect the auscultory organ is essentially different from the organs of sight and smell. The acoustic nerve is formed from ectodermic cells of the hind brain, and develops from the nervous structure that appears at its dorsal limit. On the other hand, all the membranous, cartilaginous, and osseous coverings of the labyrinth are formed from the mesodermic head-plates.

The apparatus for conducting sound which we find in the external and middle ear of mammals develops quite separately from the apparatus for the sensation of sound. It is both phylogenetically and ontogenetically an independent secondary formation, a later accession to the primary internal ear. Nevertheless, its development is not less interesting, and is explained with the same ease by comparative anatomy. In all the fishes and in the lowest Vertebrates there is no special apparatus for conducting sound, no external or middle ear; they have only a labyrinth, an internal ear, which lies within the skull. They are without the tympanum and tympanic cavity, and all its appendages. From many observations made in the last few decades it seems that many of the fishes (if not all) cannot distinguish tones; their labyrinth seems to be chiefly (if not exclusively) an organ for the sense of space (or equilibrium). If it is destroyed, the fishes lose their balance and fall. In the opinion of recent physiologists this applies also to many of the Invertebrates (including the nearer ancestors of the Vertebrates). The round vesicles which are considered to be their auscultory vesicles, and which contain an otolith, are supposed to be merely organs of the sense of space (“static vesicles or statocysts”).

Fig. 323—Primitive skull of the human embryo, four weeks old, vertical section, left half seen internally. v, z, m, h, n the five pits of the cranial cavity, in which the five cerebral vesicles lie (fore, intermediate, middle, hind, and after brains), o pear-shaped primary auscultory vesicle (appearing through), a eye (appearing through), no optic nerve, p canal of the hypophysis, t central prominence of the skull. (From Kölliker.)

The middle ear makes its first appearance in the amphibian class, where we find a tympanum, tympanic cavity, and Eustachian tube; these animals, and all terrestrial Vertebrates, certainly have the faculty of hearing. All these essential parts of the middle ear originate from the first gill-cleft and its surrounding part; in the Selachii this remains throughout life an open squirting-hole, and lies between the first and second gill-arch. In the embryo of the higher Vertebrates it closes up in the centre, and thus forms the tympanic membrane. The outlying remainder of the first gill-cleft is the rudiment of the external meatus. From its inner part we get the tympanic cavity, and, further inward still, the Eustachian tube. Connected with this is the development of the three bones of the mammal ear from the first two gill-arches; the hammer and anvil are formed from the first, the stirrup from the upper end of the second, gill-arch.

Fig. 324—The rudimentary muscles of the ear in the human skull. a raising muscle (M. attollens), b drawing muscle (M. attrahens), c withdrawing muscle (M. retrahens), d large muscle of the helix (M. helicis major), e small muscle of the helix (M. helicis minor), f muscle of the angle of the ear (M. tragicus), g anti-angular muscle (M. antitragicus). (From H. Meyer.)

Finally, the shell (pinna or concha) and external meatus (passage to the tympanum) of the outer ear are developed in a very simple fashion from the skin that borders the external aperture of the first gill-cleft. The shell rises in the shape of a circular fold of the skin, in which cartilage and muscles are afterwards formed (Figs. 313, 315). This organ is only found in the mammalian class. It is very rudimentary in the lowest section, the Monotremes. In the others it is found at very different stages of development, and sometimes of degeneration. It is degenerate in most of the aquatic mammals. The majority of them have lost it altogether—for instance, the walruses and whales and most of the seals. On the other hand, the pinna is well developed in the great majority of the Marsupials and Placentals; it receives and collects the waves of sound, and is equipped with a very elaborate muscular apparatus, by means of which the pinna can be turned freely in any direction and its shape be altered. It is well known how readily domestic animals—horses, cows, dogs, hares, etc.—point their ears and move them in different directions. Most of the apes do the same, and our earlier ape ancestors were also able to do it. But our later simian ancestors, which we have in common with the anthropoid apes, abandoned the use of these muscles, and they gradually became rudimentary and useless. However, we possess them still (Fig. 324). In fact, some men can still move their ears a little backward and forward by means of the drawing and withdrawing muscles (b and c); with practice this faculty can be much improved. But no man can now lift up his ears by the raising muscle (a), or change the shape of them by the small inner muscles (d, e, f, g). These muscles were very useful to our ancestors, but are of no consequence to us. This applies to most of the anthropoid apes as well.

We also share with the higher anthropoid apes (gorilla, chimpanzee, and orang) the characteristic form of the human outer ear, especially the folded border, the helix and the lobe. The lower apes have pointed ears, without folded border or lobe, like the other mammals. But Darwin has shown that at the upper part of the folded border there is in many men a small pointed process, which most of us do not possess. In some individuals this process is well developed. It can only be explained as the relic of the original point of the ear, which has been turned inwards in consequence of the curving of the edge. If we compare the pinna of man and the various apes in this respect, we find that they present a connected series of degenerate structures. In the common catarrhine ancestors of the anthropoids and man the degeneration set in with the folding together of the pinna. This brought about the helix of the ear, in which we find the significant angle which represents the relic of the salient point of the ear in our earlier simian ancestors. Here again, therefore, comparative anatomy enables us to trace with certainty the human ear to the similar, but more developed, organ of the lower mammals. At the same time, comparative physiology shows that it was a more or less useful implement in the latter, but it is quite useless in the anthropoids and man. The conducting of the sound has scarcely been affected by the loss of the pinna. We have also in this the explanation of the extraordinary variety in the shape and size of the shell of the ear in different men; in this it resembles other rudimentary organs.

Chapter XXVI.
EVOLUTION OF THE ORGANS OF MOVEMENT

The peculiar structure of the locomotive apparatus is one of the features that are most distinctive of the vertebrate stem. The chief part of this apparatus is formed, as in all the higher animals, by the active organs of movement, the muscles; in consequence of their contractility they have the power to draw up and shorten themselves. This effects the movement of the various parts of the body, and thus the whole body is conveyed from place to place. But the arrangement of these muscles and their relation to the solid skeleton are different in the Vertebrates from the Invertebrates.

In most of the lower animals, especially the Platodes and Vermalia, we find that the muscles form a simple, thin layer of flesh immediately underneath the skin. This muscular layer is very closely connected with the skin itself; it is the same in the Mollusc stem. Even in the large division of the Articulates, the classes of crabs, spiders, myriapods, and insects, we find a similar feature, with the difference that in this case the skin forms a solid armour—a rigid cutaneous skeleton made of chitine (and often also of carbonate of lime). This external chitine coat undergoes a very elaborate articulation both on the trunk and the limbs of the Articulates, and in consequence the muscular system also, the contractile fibres of which are attached inside the chitine tubes, is highly articulated. The Vertebrates form a direct contrast to this. In these alone a solid internal skeleton is developed, of cartilage or bone, to which the muscles are attached. This bony skeleton is a complex lever apparatus, or passive apparatus of movement. Its rigid parts, the arms of the levers, or the bones, are brought together by the actively mobile muscles, as if by drawing-ropes. This admirable locomotorium, especially its solid central axis, the vertebral column, is a special feature of the Vertebrates, and has given the name to the group.

Fig. 325—The human skeleton. From the right.
Fig. 326—The human skeleton. Front.

Fig. 327—The human vertebral column (standing upright, from the right side). (From H. Meyer.)

Fig. 328—A piece of the axial rod (chorda dorsalis), from a sheep embryo. a cuticular sheath, b cells. (From Kölliker.)

In order to get a clear idea of the chief features of the development of the human skeleton, we must first examine its composition in the adult frame (Fig. 325, the human skeleton seen from the right; Fig. 326, front view of the whole skeleton). As in other mammals, we distinguish first between the axial or dorsal skeleton and the skeleton of the limbs. The axial skeleton consists of the vertebral column (the skeleton of the trunk) and the skull (skeleton of the head); the latter is a peculiarly modified part of the former. As appendages of the vertebral column we have the ribs, and of the skull we have the hyoid bone, the lower jaw, and the other products of the gill-arches.

The skeleton of the limbs or extremities is composed of two groups of parts—the skeleton of the extremities proper and the zone-skeleton, which connects these with the vertebral column. The zone-skeleton of the arms (or fore legs) is the shoulder-zone; the zone-skeleton of the legs (or hind legs) is the pelvic zone.

The vertebral column (Fig. 327) in man is composed of thirty-three to thirty-five ring-shaped bones in a continuous series (above each other, in man’s upright position). These vertebræ are separated from each other by elastic ligaments, and at the same time connected by joints, so that the whole column forms a firm and solid, but flexible and elastic, axial skeleton, moving freely in all directions. The vertebræ differ in shape and connection at the various parts of the trunk, and we distinguish the following groups in the series, beginning at the top: Seven cervical vertebræ, twelve dorsal vertebræ, five lumbar vertebræ, five sacral vertebræ, and four to six caudal vertebræ. The uppermost, or those next to the skull, are the cervical vertebræ (Fig. 327); they have a hole in each of the lateral processes. There are seven of these vertebræ in man and almost all the other mammals, even if the neck is as long as that of the camel or giraffe, or as short as that of the mole or hedgehog. This constant number, which has few exceptions (due to adaptation), is a strong proof of the common descent of the mammals; it can only be explained by faithful heredity from a common stem-form, a primitive mammal with seven cervical vertebræ. If each species had been created separately, it would have been better to have given the long-necked mammals more, and the short-necked animals less, cervical vertebræ. Next to these come the dorsal (or pectoral) vertebræ, which number twelve to thirteen (usually twelve) in man and most of the other mammals. Each dorsal vertebra (Fig. 165) has at the side, connected by joints, a couple of ribs, long bony arches that lie in and protect the wall of the chest. The twelve pairs of ribs, together with the connecting intercostal muscles and the sternum, which joins the ends of the right and left ribs in front, form the chest (thorax). In this strong and elastic frame are the lungs, and between them the heart. Next to the dorsal vertebræ comes a short but stronger section of the column, formed of five large vertebræ. These are the lumbar vertebræ (Fig. 166); they have no ribs and no holes in the transverse processes. To these succeeds the sacral bone, which is fitted between the two halves of the pelvic zone. The sacrum is formed of five vertebræ, completely blended together. Finally, we have at the end a small rudimentary caudal column, the coccyx. This consists of a varying number (usually four, more rarely three, or five or six) of small degenerated vertebræ, and is a useless rudimentary organ with no actual physiological significance. Morphologically, however, it is of great interest as an irrefragable proof of the descent of man and the anthropoids from long-tailed apes. On no other theory can we explain the existence of this rudimentary tail. In the earlier stages of development the tail of the human embryo protrudes considerably. It afterwards atrophies; but the relic of the atrophied caudal vertebræ and of the rudimentary muscles that once moved it remains permanently. Sometimes, in fact, the external tail is preserved. The older anatomists say that the tail is usually one vertebra longer in the human female than in the male (or four against five); Steinbach says it is the reverse.

Fig. 329—Three dorsal vertebræ, from a human embryo, eight weeks old, in lateral longitudinal section. v cartilaginous vertebral body, li inter-vertebral disks, ch chorda. (From Kölliker.)

Fig. 330—A dorsal vertebra of the same embryo, in lateral transverse section. cv cartilaginous vertebral body, ch chorda, pr transverse process, a vertebral arch (upper arch), c upper end of the rib (lower arch). (From Kölliker.)

In the human vertebral column there are usually thirty-three vertebræ. It is interesting to find, however, that the number often changes, one or two vertebræ dropping out or an additional one appearing. Often, also, a mobile rib is formed at the last cervical or the first lumbar vertebra, so that there are then thirteen dorsal vertebræ, besides six cervical and four lumbar. In this way the contiguous vertebræ of the various sections of the column may take each other’s places.

In order to understand the embryology of the human vertebral column we must first carefully consider the shape and connection of the vertebræ. Each vertebra has, in general, the shape of a seal-ring (Figs. 164–166). The thicker portion, which is turned towards the ventral side, is called the body of the vertebra, and forms a short osseous disk; the thinner part forms a semi-circular arch, the vertebral arch, and is turned towards the back. The arches of the successive vertebræ are connected by thin intercrural ligaments in such a way that the cavity they collectively enclose represents a long canal. In this vertebral canal we find the trunk part of the central nervous system, the spinal cord. Its head part, the brain, is enclosed by the skull, and the skull itself is merely the uppermost part of the vertebral column, distinctively modified. The base or ventral side of the vesicular cranial capsule corresponds originally to a number of developed vertebral bodies; its vault or dorsal side to their combined upper vertebral arches.

While the solid, massive bodies of the vertebræ represent the real central axis of the skeleton, the dorsal arches serve to protect the central marrow they enclose. But similar arches develop on the ventral side for the protection of the viscera in the breast and belly. These lower or ventral vertebral arches, proceeding from the ventral side of the vertebral bodies, form, in many of the lower Vertebrates, a canal in which the large blood-vessels are enclosed on the lower surface of the vertebral column (aorta and caudal vein). In the higher Vertebrates the majority of these vertebral arches are lost or become rudimentary. But at the thoracic section of the column they develop into independent strong osseous arches, the ribs (costæ). In reality the ribs are merely large and independent lower vertebral arches, which have lost their original connection with the vertebral bodies.

Fig. 331—Intervertebral disk of a new-born infant, transverse section. a rest of the chorda. (From Kölliker.)

If we turn from this anatomic survey of the composition of the column to the question of its development, I may refer the reader to earlier pages with regard to the first and most important points (pp. 145–148). It will be remembered that in the human embryo and that of the other vertebrates we find at first, instead of the segmented column, only a simple unarticulated cartilaginous rod. This solid but flexible and elastic rod is the axial rod (or the chorda dorsalis). In the lowest Vertebrate, the Amphioxus, it retains this simple form throughout life, and permanently represents the whole internal skeleton (Fig. 210 i). In the Tunicates, also, the nearest Invertebrate relatives of the Vertebrates, we meet the same chorda—transitorily in the passing larva tail of the Ascidia, permanently in the Copelata (Fig. 225 c). Undoubtedly both the Tunicates and Acrania have inherited the chorda from a common unsegmented stem-form; and these ancient, long-extinct ancestors of all the chordonia are our hypothetical Prochordonia.

Long before there is any trace of the skull, limbs, etc., in the embryo of man or any of the higher Vertebrates—at the early stage in which the whole body is merely a sole-shaped embryonic shield—there appears in the middle line of the shield, directly under the medullary furrow, the simple chorda. (Cf. Figs. 131–135 ch). It follows the long axis of the body in the shape of a cylindrical axial rod of elastic but firm composition, equally pointed at both ends. In every case the chorda originates from the dorsal wall of the primitive gut; the cells that compose it (Fig. 328 b) belong to the entoderm (Figs. 216–221). At an early stage the chorda develops a transparent structureless sheath, which is secreted from its cells (Fig. 328 a). This chordalemma is often called the “inner chorda-sheath,” and must not be confused with the real external sheath, the mesoblastic perichorda.

Fig. 332—Human skull.

But this unsegmented primary axial skeleton is soon replaced by the segmented secondary axial skeleton, which we know as the vertebral column. The provertebral plates (Fig. 124 s) differentiate from the innermost, median part of the visceral layer of the cœlom-pouches at each side of the chorda. As they grow round the chorda and enclose it they form the skeleton plate or skeletogenetic layer—that is to say, the skeleton-forming stratum of cells, which provides the mobile foundation of the permanent vertebral column and skull (scleroblast). In the head-half of the embryo the skeletal plate remains a continuous, simple, undivided layer of tissue, and presently enlarges into a thin-walled capsule enclosing the brain, the primordial skull. In the trunk-half the provertebral plate divides into a number of homogeneous, cubical, successive pieces; these are the several primitive vertebræ. They are not numerous at first, but soon increase as the embryo grows longer (Figs. 153–155).

Fig. 333—Skull of a new-born child. (From Kollmann.) Above, in the three bones of the roof of the skull, we see the lines that radiate from the central points of ossification; in front, the frontal bone; behind, the occipital bone; between the two the large parietal bone, p. s the scurf bone, w mastoid fontanelle, f petrous bone, t tympanic bone, l lateral part, b bulla, j cheek-bone, a large wing of cuneiform bone, k fontanelle of cuneiform bone.

In all the Craniotes the soft, indifferent cells of the mesoderm, which originally compose the skeletal plate, are afterwards converted for the most part into cartilaginous cells, and these secrete a firm and elastic intercellular substance between them, and form cartilaginous tissue. Like most of the other parts of the skeleton, the membranous rudiments of the vertebræ soon pass into a cartilaginous state, and in the higher Vertebrates this is afterwards replaced by the hard osseous tissue with its characteristic stellate cells (Fig. 6). The primary axial skeleton remains a simple chorda throughout life in the Acrania, the Cyclostomes, and the lowest fishes. In most of the other Vertebrates the chorda is more or less replaced by the cartilaginous tissue of the secondary perichorda that grows round it. In the lower Craniotes (especially the fishes) a more or less considerable part of the chorda is preserved in the bodies of the vertebræ. In the mammals it disappears for the most part. By the end of the second month in the human embryo the chorda is merely a slender thread, running through the axis of the thick, cartilaginous vertebral column (Figs. 182 ch, 329 ch). In the cartilaginous vertebral bodies themselves, which afterwards ossify, the slender remnant of the chorda presently disappears (Fig. 330 ch). But in the elastic inter-vertebral disks, which develop from the skeletal plate between each pair of vertebral bodies (Fig. 329 li), a relic of the chorda remains permanently. In the new-born child there is a large pear-shaped cavity in each intervertebral disk, filled with a gelatinous mass of cells (Fig. 331 a).

Fig. 334—Head-skeleton of a primitive fish. n nasal pit, eth cribriform bone region, orb orbit of eye, la wall of auscultory labyrinth, occ occipital region of primitive skull, cv vertebral column, a fore, bc hind-lip cartilage, o primitive upper jaw (palato-quadratum), u primitive lower jaw, II hyaloid bone, III–VIII first to sixth branchial arches. (From Gegenbaur.)

Though less sharply defined, this gelatinous nucleus of the elastic cartilaginous disks persists throughout life in the mammals, but in the birds and most reptiles the last trace of the chorda disappears. In the subsequent ossification of the cartilaginous vertebra the first deposit of bony matter (“first osseous nucleus”) takes place in the vertebral body immediately round the remainder of the chorda, and soon displaces it altogether. Then there is a special osseous nucleus formed in each half of the vertebral arch. The ossification does not reach the point at which the three nuclei are joined until after birth. In the first year the two osseous halves of the arches unite; but it is much later—in the second to the eighth year— that they connect with the osseous vertebral bodies.

Fig. 335—Roofs of the skulls of nine Primates (Cattarrhines), seen from above and reduced to a common size. 1 European, 2 Brazilian, 3 Pithecanthropus, 4 Gorilla, 5 Chimpanzee, 6 Orang, 7 Gibbon, 8 Tailed ape, 9 Baboon.

The bony skull (cranium), the head-part of the secondary axial skeleton, develops in just the same way as the vertebral column. The skull forms a bony envelope for the brain, just as the vertebral canal does for the spinal cord; and as the brain is only a peculiarly differentiated part of the head, while the spinal cord represents the longer trunk-section of the originally homogeneous medullary tube, we shall expect to find that the osseous coat of the one is a special modification of the osseous envelope of the other. When we examine the adult human skull in itself (Fig. 332), it is difficult to conceive how it can be merely the modified fore part of the vertebral column. It is an elaborate and extensive bony structure, composed of no less than twenty bones of different shapes and sizes. Seven of them form the spacious shell that surrounds the brain, in which we distinguish the solid ventral base below and the curved dorsal vault above. The other thirteen bones form the facial skull, which is especially the bony envelope of the higher sense-organs, and at the same time encloses the entrance of the alimentary canal. The lower jaw is articulated at the base of the skull (usually regarded as the XXI cranial bone). Behind the lower jaw we find the hyoid bone at the root of the tongue, also formed from the gill-arches, and a part of the lower arches that have developed as “head-ribs” from the ventral side of the base of the cranium.

Fig. 336—Skeleton of the breast-fin of Ceratodus (biserial feathered skeleton). A, B, cartilaginous series of the fin-stem. rr cartilaginous fin-radii. (From Gunther.)
Fig. 337—Skeleton of the breast-fin of an early Selachius (Acanthias). The radii of the median fin-border (B) have disappeared for the most part; a few only (R) are left. R, R, radii of the lateral fin-border, mt metapterygium, ms mesopterygium, p propterygium. (From Gegenbaur.)
Fig. 338—Skeleton of the breast-fin of a young Selachius. The radii of the median fin-border have wholly disappeared. The shaded part on the right is the section that persists in the five-fingered hand of the higher Vertebrates. (b the three basal pieces of the fin: mt metapterygium, rudiment of the humerus, ms mesopterygium, p propterygium.) (From Gegenbaur.)

Although the fully-developed skull of the higher Vertebrates, with its peculiar shape, its enormous size, and its complex composition, seems to have nothing in common with the ordinary vertebræ, nevertheless even the older comparative anatomists came to recognise at the end of the eighteenth century that it is really nothing else originally than a series of modified vertebræ. When Goethe in 1790 “picked up the skull of a slain victim from the sand of the Jewish cemetery at Venice, he noticed at once that the bones of the face also could be traced to vertebræ (like the three hind-most cranial vertebræ).” And when Oken (without knowing anything of Goethe’s discovery) found at Ilenstein, “a fine bleached skull of a hind, the thought flashed across him like lightning: ‘It is a vertebral column.’”

This famous vertebral theory of the skull has interested the most distinguished zoologists for more than a century: the chief representatives of comparative anatomy have devoted their highest powers to the solution of the problem, and the interest has spread far beyond their circle. But it was not until 1872 that it was happily solved, after seven years’ labour, by the comparative anatomist who surpassed all other experts of this science in the second half of the nineteenth century by the richness of his empirical knowledge and the acuteness and depth of his philosophic speculations. Carl Gegenbaur has shown, in his classic Studies of the Comparative Anatomy of the Vertebrates (third section), that we find the most solid foundation for the vertebral theory of the skull in the head-skeleton of the Selachii. Earlier anatomists had wrongly started from the mammal skull, and had compared the several bones that compose it with the several parts of the vertebra (Fig. 333) they thought they could prove in this way that the fully-formed mammal skull was made of from three to six vertebræ.

Fig. 339—Skeleton of the fore leg of an amphibian. h upper-arm (humerus), ru lower arm (r radius, u ulna), rcicu′, wrist-bones of first series (r radiale, i intermedium, c centrale, u′ ulnare). 1, 2, 3, 4, 5 wrist-bones of the second series. (From Gegenbaur.)
Fig. 340—Skeleton of gorilla’s hand. (From Huxley.)
Fig. 341—Skeleton of human hand, back. (From Meyer.)

The older theory was refuted by simple and obvious facts, which were first pointed out by Huxley. Nevertheless, the fundamental idea of it—the belief that the skull is formed from the head-part of the perichordal axial skeleton, just as the brain is from the simple medullary tube, by differentiation and modification—remained. The work now was to discover the proper way of supplying this philosophic theory with an empirical foundation, and it was reserved for Gegenbaur to achieve this. He first opened out the phylogenetic path which here, as in all morphological questions, leads most confidently to the goal. He showed that the primitive fishes (Figs. 249–251), the ancestors of all the Gnathostomes, still preserve permanently in the form of their skull the structure out of which the transformed skull of the higher Vertebrates, including man, has been evolved. He further showed that the branchial arches of the Selachii prove that their skull originally consisted of a large number of (at least nine or ten) provertebræ, and that the cerebral nerves that proceed from the base of the brain entirely confirm this. These cerebral nerves are (with the exception of the first and second pair, the olfactory and optic nerves) merely modifications of spinal nerves, and are essentially similar to them in their peripheral expansion. The comparative anatomy of these cerebral nerves, their origin and their expansion, furnishes one of the strongest arguments for the new vertebral theory of the skull.

Fig. 342—Skeleton of the hand or fore foot of six mammals. I man, II dog, III pig, IV ox, V tapir, VI horse. r radius, u ulna, a scaphoideum, b lunare, a triquetrum, d trapezium, e trapezoid, f capitatum, g hamatum, p pisiforme. 1 thumb, 2 index finger, 3 middle finger, 4 ring finger, 5 little finger. (From Gegenbaur.)

We have not space here to go into the details of Gegenbaur’s theory of the skull. I must be content to refer the reader to the great work I have mentioned, in which it is thoroughly established from the empirico-philosophical point of view. He has also given a comprehensive and up-to-date treatment of the subject in his Comparative Anatomy of the Vertebrates (1898). Gegenbaur indicates as original “cranial ribs,” or “lower arches of the cranial vertebræ,” at each side of the head of the Selachii (Fig. 334), the following pairs of arches: I and II, two lip-cartilages, the anterior (a) of which is composed of an upper piece only, the posterior (bc) from an upper and lower piece; III, the maxillary arches, also consisting of two pieces on each side—the primitive upper jaw (os palato-quadratum, o) and the primitive lower jaw (u); IV, the hyaloid bone (II); finally, V–X, six branchial arches in the narrower sense (III–VIII). From the anatomic features of these nine to ten cranial ribs or “lower vertebral arches” and the cranial nerves that spread over them, it is clear that the apparently simple cartilaginous primitive skull of the Selachii was originally formed from so many (at least nine) somites or provertebræ. The blending of these primitive segments into a single capsule is, however, so ancient that, in virtue of the law of curtailed heredity, the original division seems to have disappeared; in the embryonic development it is very difficult to detect it in isolated traces, and in some respects quite impossible. It is claimed that several (three to six) traces of provertebræ have been discovered in the anterior (pre-chordal) part of the Selachii-skull; this would bring up the number of cranial somites to twelve or sixteen, or even more.

In the primitive skull of man (Fig. 323) and the higher Vertebrates, which has been evolved from that of the Selachii, five consecutive sections are discoverable at a certain early period of development, and one might be induced to trace these to five primitive vertebræ; but these sections are due entirely to adaptation to the five primitive cerebral vesicles, and correspond, like these, to a large number of metamera. That we have in the primitive skull of the mammals a greatly modified and transformed organ, and not at all a primitive formation, is clear from the circumstance that its original soft membranous form only assumes the cartilaginous character for the most part at the base and the sides, and remains membranous at the roof. At this part the bones of the subsequent osseous skull develop as external coverings over the membranous structure, without an intermediate cartilaginous stage, as there is at the base of the skull. Thus a large part of the cranial bones develop originally as covering bones from the corium, and only secondarily come into close touch with the primitive skull (Fig. 333). We have previously seen how this very rudimentary beginning of the skull in man is formed ontogenetically from the “head-plates,” and thus the fore end of the chorda is enclosed in the base of the skull. (Cf. Fig. 145 and pp. 138, 144, and 149.)

Figs. 343–345—Arm and hand of three anthropoids. Fig. 343—Chimpanzee (Anthropithecus niger). Fig. 344—Veddah of Ceylon (Homo veddalis). Fig. 345—European (Homo mediterraneus). (From Paul and Fritz Sarasin.)

The phylogeny of the skull has made great progress during the last three decades through the joint attainments of comparative anatomy, ontogeny, and paleontology. By the judicious and comprehensive application of the phylogenetic method (in the sense of Gegenbaur) we have found the key to the great and important problems that arise from the thorough comparative study of the skull. Another school of research, the school of what is called “exact craniology” (in the sense of Virchow), has, meantime, made fruitless efforts to obtain this result. We may gratefully acknowledge all that this descriptive school has done in the way of accurately describing the various forms and measurements of the human skull, as compared with those of other mammals. But the vast empirical material that it has accumulated in its extensive literature is mere dead and sterile erudition until it is vivified and illumined by phylogenetic speculation.

Virchow confined himself to the most careful analysis of large numbers of human skulls and those of anthropoid mammals. He saw only the differences between them, and sought to express these in figures.

Fig. 346—Transverse section of a fish’s tail (from the tunny). (From Johannes Müller.) a upper (dorsal) lateral muscles, a′, b′ lower (ventral) lateral muscles, d vertebral bodies, b sections of incomplete conical mantle, B attachment lines of the inter-muscular ligaments (from the side).

Without adducing a single solid reason, or offering any alternative explanation, he rejected evolution as an unproved hypothesis. He played a most unfortunate part in the controversy as to the significance of the fossil human skulls of Spy and Neanderthal, and the comparison of them with the skull of the Pithecanthropus (Fig. 283). All the interesting features of these skulls that clearly indicated the transition from the anthropoid to the man were declared by Virchow to be chance pathological variations. He said that the roof of the skull of Pithecanthropus (Fig. 335, 3) must have belonged to an ape, because so pronounced an orbital stricture (the horizontal constriction between the outer edge of the eye-orbit and the temples) is not found in any human being. Immediately afterwards Nehring showed in the skull of a Brazilian Indian (Fig. 335, 2), found in the Sambaquis of Santos, that this stricture can be even deeper in man than in many of the apes. It is very instructive in this connection to compare the roofs of the skulls (seen from above) of different primates. I have, therefore, arranged nine such skulls in Fig. 335, and reduced them to a common size.

We turn now to the branchial arches, which were regarded even by the earlier natural philosophers as “head-ribs.” (Cf. Figs. 167–170). Of the four original gill-arches of the mammals the first lies between the primitive mouth and the first gill-cleft. From the base of this arch is formed the upper-jaw process, which joins with the inner and outer nasal processes on each side, in the manner we have previously explained, and forms the chief parts of the skeleton of the upper jaw (palate bone, pterygoid bone, etc.) (Cf. p. 284.) The remainder of the first branchial arch, which is now called, by way of contrast, the “upper-jaw process,” forms from its base two of the ear-ossicles (hammer and anvil), and as to the rest is converted into a long strip of cartilage that is known, after its discoverer, as “Meckel’s cartilage,” or the promandibula. At the outer surface of the latter is formed from the cellular matter of the corium, as covering or accessory bone, the permanent bony lower jaw. From the first part or base of the second branchial arch we get, in the mammals, the third ossicle of the ear, the stirrup; and from the succeeding parts we get (in this order) the muscle of the stirrup, the styloid process of the temporal bone, the styloid-hyoid ligament, and the little horn of the hyoid bone. The third branchial arch is only cartilaginous at the foremost part, and here the body of the hyoid bone and its larger horn are formed at each side by the junction of its two halves. The fourth branchial arch is only found transitorily in the mammal embryo as a rudimentary organ, and does not develop special parts; and there is no trace in the embryo of the higher Vertebrates of the posterior branchial arches (fifth and sixth pair), which are permanent in the Selachii. They have been lost long ago. Moreover, the four gill-clefts of the human embryo are only interesting as rudimentary organs, and they soon close up and disappear. The first alone (between the first and second branchial arches) has any permanent significance; from it are developed the tympanic cavity and the Eustachian tube. (Cf. Figs. 169, 320.)

It was Carl Gegenbaur again who solved the difficult problem of tracing the skeleton of the limbs of the Vertebrates to a common type. Few parts of the vertebrate body have undergone such infinitely varied modifications in regard to size, shape, and adaptation of structure as the limbs or extremities; yet we are in a position to reduce them all to the same hereditary standard. We may generally distinguish three groups among the Vertebrates in relation to the formation of their limbs. The lowest and earliest Vertebrates, the Acrania and Cyclostomes, had, like their invertebrate ancestors, no pairs of limbs, as we see in the Amphioxus and the Cyclostomes to-day (Figs. 210, 247). The second group is formed of the two classes of the true fishes and the Dipneusts; here there are always two pairs of limbs at first, in the shape of many-toed fins—one pair of breast-fins or fore legs, and one pair of belly-fins or hind legs (Figs. 248–259). The third group comprises the four higher classes of Vertebrates—the amphibia, reptiles, birds, and mammals; in these quadrupeds there are at first the same two pairs of limbs, but in the shape of five-toed feet. Frequently we find less than five toes, and sometimes the feet are wholly atrophied (as in the serpents). But the original stem-form of the group had five toes or fingers before and behind (Figs. 263–265).

The true primitive form of the pairs of limbs, such as they were found in the primitive fishes of the Silurian period, is preserved for us in the Australian dipneust, the remarkable Ceratodus (Fig. 257). Both the breast-fin and the belly-fin are flat oval paddles, in which we find a biserial cartilaginous skeleton (Fig. 336). This consists, firstly, of a much segmented fin-rod or “stem” (A, B), which runs through the fin from base to tip; and secondly of a double row of thin articulated fin-radii (r, r), which are attached to both sides of the fin-rod, like the feathers of a feathered leaf. This primitive fin, which Gegenbaur first recognised, is attached to the vertebral column by a simple zone in the shape of a cartilaginous arch. It has probably originated from the branchial arches.[^[31]]

[31] While Gegenbaur derives the fins from two pairs of posterior separated branchial arches, Balfour holds that they have been developed from segments of a pair of originally continuous lateral fins or folds of the skin.)

We find the same biserial primitive fin more or less preserved in the fossilised remains of the earliest Selachii (Fig. 248), Ganoids (Fig. 253), and Dipneusts (Fig. 256). It is also found in modified form in some of the actual sharks and pikes. But in the majority of the Selachii it has already degenerated to the extent that the radii on one side of the fin-rod have been partly or entirely lost, and are retained only on the other (Fig. 337). We thus get the uniserial fin, which has been transmitted from the Selachii to the rest of the fishes (Fig. 338).

Gegenbaur has shown how the five-toed leg of the Amphibia, that has been inherited by the three classes of Amniotes, was evolved from the uniserial fish-fin.[^[32]]

[32] The limb of the four higher classes of Vertebrates is now explained in the sense that the original fin-rod passes along its outer (ulnar or fibular) side, and ends in the fifth toe. It was formerly believed to go along the inner (radial or tibial) side, and end in the first toe, as Fig. 339 shows.) In the dipneust ancestors of the Amphibia the radii gradually atrophy, and are lost, for the most part, on the other side of the fin-rod as well (the lighter cartilages in Fig. 338). Only the four lowest radii (shaded in the illustration) are preserved; and these are the four inner toes of the foot (first to fourth). The little or fifth toe is developed from the lower end of the fin-rod. From the middle and upper part of the fin-rod was developed the long stem of the limb—the important radius and ulna (Fig. 339 r and u) and humerus (h) of the higher Vertebrates.

Fig. 347—Human skeleton. (Cf. Figure 326.)
Fig. 348—Skeleton of the giant gorilla. (Cf. Figure 209.)

In this way the five-toed foot of the Amphibia, which we first meet in the Carboniferous Stegocephala (Fig. 260), and which was inherited from them by the reptiles on one side and the mammals on the other, was formed by gradual degeneration and differentiation from the many-toed fish-fin (Fig. 341). The reduction of the radii to four was accompanied by a further differentiation of the fin-rod, its transverse segmentation into upper and lower halves, and the formation of the zone of the limb, which is composed originally of three limbs before and behind in the higher Vertebrates. The simple arch of the original shoulder-zone divides on each side into an upper (dorsal) piece, the shoulder-blade (scapula), and a lower (ventral) piece; the anterior part of the latter forms the primitive clavicle (procoracoideum), and the posterior part the coracoideum. In the same way the simple arch of the pelvic zone breaks up into an upper (dorsal) piece, the iliac-bone (os ilium), and a lower (ventral) piece; the anterior part of the latter forms the pubic bone (os pubis), and the posterior the ischial bone (os ischii).

There is also a complete agreement between the fore and hind limb in the stem or shaft. The first section of the stem is supported by a single strong bone—the humerus in the fore, the femur in the hind limb. The second section contains two bones: in front the radius (r) and ulna (u), behind the tibia and fibula. (Cf. the skeletons in Figs. 260, 265, 270, 278–282, and 348.) The succeeding numerous small bones of the wrist (carpus) and ankle (tarsus) are also similarly arranged in the fore and hind extremities, and so are the five bones of the middle-hand (metacarpus) and middle-foot (metatarsus). Finally, it is the same with the toes themselves, which have a similar characteristic composition from a series of bony pieces before and behind. We find a complete parallel in all the parts of the fore leg and the hind leg.

When we thus learn from comparative anatomy that the skeleton of the human limbs is composed of just the same bones, put together in the same way, as the skeleton in the four higher classes of Vertebrates, we may at once infer a common descent of them from a single stem-form. This stem-form was the earliest amphibian that had five toes on each foot. It is particularly the outer parts of the limbs that have been modified by adaptation to different conditions. We need only recall the immense variations they offer within the mammal class. We have the slender legs of the deer and the strong springing legs of the kangaroo, the climbing feet of the sloth and the digging feet of the mole, the fins of the whale and the wings of the bat. It will readily be granted that these organs of locomotion differ as much in regard to size, shape, and special function as can be conceived. Nevertheless, the bony skeleton is substantially the same in every case. In the different limbs we always find the same characteristic bones in essentially the same rigidly hereditary connection; this is as splendid a proof of the theory of evolution as comparative anatomy can discover in any organ of the body. It is true that the skeleton of the limbs of the various mammals undergoes many distortions and degenerations besides the special adaptations (Fig. 342). Thus we find the first finger or the thumb atrophied in the fore-foot (or hand) of the dog (II). It has entirely disappeared in the pig (III) and tapir (V). In the ruminants (such as the ox, IV) the second and fifth toes are also atrophied, and only the third and fourth are well developed (VI, 3). Nevertheless, all these different fore-feet, as well as the hand of the ape (Fig. 340) and of man (Fig. 341), were originally developed from a common pentadactyle stem-form. This is proved by the rudiments of the degenerated toes, and by the similarity of the arrangement of the wrist-bones in all the pentanomes (Fig. 342 a–p).

If we candidly compare the bony skeleton of the human arm and hand with that of the nearest anthropoid apes, we find an almost perfect identity. This is especially true of the chimpanzee. In regard to the proportions of the various parts, the lowest living races of men (the Veddahs of Ceylon, Fig. 344) are midway between the chimpanzee (Fig. 343) and the European (Fig. 345). More considerable are the differences in structure and the proportions of the various parts between the different genera of anthropoid apes (Figs. 278–282); and still greater is the morphological distance between these and the lowest apes (the Cynopitheca). Here, again, impartial and thorough anatomic comparison confirms the accuracy of Huxley’s pithecometra principle p. 171.

The complete unity of structure which is thus revealed by the comparative anatomy of the limbs is fully confirmed by their embryology. However different the extremities of the four-footed Craniotes may be in their adult state, they all develop from the same rudimentary structure. In every case the first trace of the limb in the embryo is a very simple protuberance that grows out of the side of the hyposoma. These simple structures develop directly into fins in the fishes and Dipneusts by differentiation of their cells. In the higher classes of Vertebrates each of the four takes the shape in its further growth of a leaf with a stalk, the inner half becoming narrower and thicker and the outer half broader and thinner. The inner half (the stalk of the leaf) then divides into two sections—the upper and lower parts of the limb. Afterwards four shallow indentations are formed at the free edge of the leaf, and gradually deepen; these are the intervals between the five toes (Fig. 174). The toes soon make their appearance. But at first all five toes, both of fore and hind feet, are connected by a thin membrane like a swimming-web; they remind us of the original shaping of the foot as a paddling fin. The further development of the limbs from this rudimentary structure takes place in the same way in all the Vertebrates according to the laws of heredity.

The embryonic development of the muscles, or active organs of locomotion, is not less interesting than that of the skeleton, or passive organs. But the comparative anatomy and ontogeny of the muscular system are much more difficult and inaccessible, and consequently have hitherto been less studied. We can therefore only draw some general phylogenetic conclusions therefrom.

It is incontestable that the musculature of the Vertebrates has been evolved from that of lower Invertebrates; and among these we have to consider especially the unarticulated Vermalia. They have a simple cutaneous muscular layer, developing from the mesoderm. This was afterwards replaced by a pair of internal lateral muscles, that developed from the middle wall of the cœlom-pouches; we still find the first rudiments of the muscles arising from the muscle-plate of these in the embryos of all the Vertebrates (cf. Figs. 124, 158–160, 222–224 mp). In the unarticulated stem-forms of the Chordonia, which we have called the Prochordonia, the two cœlom-pouches, and therefore also the muscle-plates of their walls, were not yet segmented. A great advance was made in the articulation of them, as we have followed it step by step in the Amphioxus (Figs. 124, 158). This segmentation of the muscles was the momentous historical process with which vertebration, and the development of the vertebrate stem, began. The articulation of the skeleton came after this segmentation of the muscular system, and the two entered into very close correlation.

The episomites or dorsal cœlom-pouches of the Acrania, Cyclostomes, and Selachii (Fig. 161 h) first develop from their inner or median wall (from the cell-layer that lies directly on the skeletal plate [sk] and the medullary tube [nr]) a strong muscle-plate (mp). By dorsal growth (w) it also reaches the external wall of the cœlom-pouches, and proceeds from the dorsal to the ventral wall. From these segmental muscle-plates, which are chiefly concerned in the segmentation of the Vertebrates, proceed the lateral muscles of the stem, as we find in the simplest form in the Amphioxus (Fig. 210). By the formation of a horizontal frontal septum they divide on each side into an upper and lower series of myotomes, dorsal and ventral lateral muscles. This is seen with typical regularity in the transverse section of the tail of a fish (Fig. 346). From these earlier lateral muscles of the trunk develop the greater part of the subsequent muscles of the trunk, and also the much later “muscular buds” of the limbs.[^[33]]

[33] The ontogeny of the muscles is mostly cenogenetic. The greater part of the muscles of the head (or the visceral muscles) belong originally to the hyposoma of the vertebrate organism, and develop from the wall of the hyposomites or ventral cœlom-pouches. This also applies originally to the primary muscles of the limbs, as these too belong phylogenetically to the hyposoma. (Cf. Chapter XIV.)

Chapter XXVII.
THE EVOLUTION OF THE ALIMENTARY SYSTEM

The chief of the vegetal organs of the human frame, to the evolution of which we now turn our attention, is the alimentary canal. The gut is the oldest of all the organs of the metazoic body, and it leads us back to the earliest age of the formation of organs—to the first section of the Laurentian period. As we have already seen, the result of the first division of labour among the homogeneous cells of the earliest multicellular animal body was the formation of an alimentary cavity. The first duty and first need of every organism is self-preservation. This is met by the functions of the nutrition and the covering of the body. When, therefore, in the primitive globular Blastæa the homogeneous cells began to effect a division of labour, they had first to meet this twofold need. One half were converted into alimentary cells and enclosed a digestive cavity, the gut. The other half became covering cells, and formed an envelope round the alimentary tube and the whole body. Thus arose the primary germinal layers—the inner, alimentary, or vegetal layer, and the outer, covering, or animal layer. (Cf. pp. 214–17.)

When we try to construct an animal frame of the simplest conceivable type, that has some such primitive alimentary canal and the two primary layers constituting its wall, we inevitably come to the very remarkable embryonic form of the gastrula, which we have found with extraordinary persistence throughout the whole range of animals, with the exception of the unicellulars—in the Sponges, Cnidaria, Platodes, Vermalia, Molluscs, Articulates, Echinoderms, Tunicates, and Vertebrates. In all these stems the gastrula recurs in the same very simple form. It is certainly a remarkable fact that the gastrula is found in various animals as a larva-stage in their individual development, and that this gastrula, though much disguised by cenogenetic modifications, has everywhere essentially the same palingenetic structure (Figs. 30–35). The elaborate alimentary canal of the higher animals develops ontogenetically from the same simple primitive gut of the gastrula.

This gastræa theory is now accepted by nearly all zoologists. It was first supported and partly modified by Professor Ray-Lankester; he proposed three years afterwards (in his essay on the development of the Molluscs, 1875) to give the name of archenteron to the primitive gut and blastoporus to the primitive mouth.

Before we follow the development of the human alimentary canal in detail, it is necessary to say a word about the general features of its composition in the fully-developed man. The mature alimentary canal in man is constructed in all its main features like that of all the higher mammals, and particularly resembles that of the Catarrhines, the narrow-nosed apes of the Old World. The entrance into it, the mouth, is armed with thirty-two teeth, fixed in rows in the upper and lower jaws. As we have seen, our dentition is exactly the same as that of the Catarrhines, and differs from that of all other animals p. 257. Above the mouth-cavity is the double nasal cavity; they are separated by the palate-wall. But we saw that this separation is not there from the first, and that originally there is a common mouth-nasal cavity in the embryo; and this is only divided afterwards by the hard palate into two—the nasal cavity above and that of the mouth below (Fig. 311).

At the back the cavity of the mouth is half closed by the vertical curtain that we call the soft palate, in the middle of which is the uvula. A glance into a mirror with the mouth wide open will show its shape. The uvula is interesting because, besides man, it is only found in the ape. At each side of the soft palate are the tonsils. Through the curved opening that we find underneath the soft palate we penetrate into the gullet or pharynx behind the mouth-cavity. Into this opens on either side a narrow canal (the Eustachian tube), through which there is direct communication with the tympanic cavity of the ear (Fig. 320 e). The pharynx is continued in a long, narrow tube, the œsophagus ( sr). By this the food passes into the stomach when masticated and swallowed. Into the gullet also opens, right above, the trachea ( lr), that leads to the lungs. The entrance to it is covered by the epiglottis, over which the food slides. The cartilaginous epiglottis is found only in the mammals, and has developed from the fourth branchial arch of the fishes and amphibia. The lungs are found, in man and all the mammals, to the right and left in the pectoral cavity, with the heart between them. At the upper end of the trachea there is, under the epiglottis, a specially differentiated part, strengthened by a cartilaginous skeleton, the larynx. This important organ of human speech also develops from a part of the alimentary canal. In front of the larynx is the thyroid gland, which sometimes enlarges and forms goitre.

The œsophagus descends into the pectoral cavity along the vertebral column, behind the lungs and the heart, pierces the diaphragm, and enters the visceral cavity. The diaphragm is a membrano-muscular partition that completely separates the thoracic from the abdominal cavity in all the mammals (and these alone). This separation is not found in the beginning; there is at first a common breast-belly cavity, the cœloma or pleuro-peritoneal cavity. The diaphragm is formed later on as a muscular horizontal partition between the thoracic and abdominal cavities. It then completely separates the two cavities, and is only pierced by several organs that pass from the one to the other. One of the chief of these organs is the œsophagus. After this has passed through the diaphragm, it expands into the gastric sac in which digestion chiefly takes place. The stomach of the adult man (Fig. 349) is a long, somewhat oblique sac, expanding on the left into a blind sac, the fundus of the stomach ( b′), but narrowing on the right, and passing at the pylorus ( e) into the small intestine. At this point there is a valve, the pyloric valve ( d), between the two sections of the canal; it opens only when the pulpy food passes from the stomach into the intestine. In man and the higher Vertebrates the stomach itself is the chief organ of digestion, and is especially occupied with the solution of the food; this is not the case in many of the lower Vertebrates, which have no stomach, and discharge its function by a part of the gut farther on. The muscular wall of the stomach is comparatively thick; it has externally strong muscles that accomplish the digestive movements, and internally a large quantity of small glands, the peptic glands, which secrete the gastric juice.

Fig. 349—Human stomach and duodenum, longitudinal section. a cardiac (end of œsophagus), b fundus (blind sac of the left side), c pylorus-fold, d pylorus-valves, e pylorus-cavity, fgh duodenum, i entrance of the gall-duct and the pancreatic duct. (From Meyer.)

Next to the stomach comes the longest section of the alimentary canal, the middle gut or small intestine. Its chief function is to absorb the peptonised fluid mass of food, or the chyle, and it is subdivided into several sections, of which the first (next to the stomach) is called the duodenum (Fig. 349 fgh). It is a short, horseshoe-shaped loop of the gut. The largest glands of the alimentary canal open into it—the liver, the chief digestive gland, that secretes the gall, and the pancreas, which secretes the pancreatic juice. The two glands pour their secretions, the bile and pancreatic juice, close together into the duodenum ( i). The opening of the gall-duct is of particular phylogenetic importance, as it is the same in all the Vertebrates, and indicates the principal point of the hepatic or trunk-gut (Gegenbaur). The liver, phylogenetically older than the stomach, is a large gland, rich in blood, in the adult man, immediately under the diaphragm on the left side, and separated by it from the lungs. The pancreas lies a little further back and more to the left. The remaining part of the small intestine is so long that it has to coil itself in many folds in order to find room in the narrow space of the abdominal cavity. It is divided into the jejunum above and the ileum below. In the last section of it is the part of the small intestine at which in the embryo the yelk-sac opens into the gut. This long and thin intestine then passes into the large intestine, from which it is cut off by a special valve. Immediately behind this “Bauhin-valve” the first part of the large intestine forms a wide, pouch-like structure, the cæcum. The atrophied end of the cæcum is the famous rudimentary organ, the vermiform appendix. The large intestine ( colon) consists of three parts—an ascending part on the right, a transverse middle part, and a descending part on the left. The latter finally passes through an S-shaped bend into the last section of the alimentary canal, the rectum, which opens behind by the anus. Both the large and small intestines are equipped with numbers of small glands, which secrete mucous and other fluids.

Fig. 350—Median section of the head of a hare-embryo, one-fourth of an inch in length. (From Mihalcovics.) The deep mouth-cleft ( hp) is separated by the membrane of the throat ( rh) from the blind cavity of the head-gut ( kd). hz heart, ch chorda, hp the point at which the hypophysis develops from the mouth-cleft, vh ventricle of the cerebrum, v3 , third ventricle (intermediate brain), v4 fourth ventricle (hind brain), ck spinal canal.

For the greater part of its length the alimentary canal is attached to the inner dorsal surface of the abdominal cavity, or to the lower surface of the vertebral column. The fixing is accomplished by means of the thin membranous plate that we call the mesentery.

Although the fully-formed alimentary canal is thus a very elaborate organ, and although in detail it has a quantity of complex structural features into which we cannot enter here, nevertheless the whole complicated structure has been historically evolved from the very simple form of the primitive gut that we find in our gastræad-ancestors, and that every gastrula brings before us to-day. We have already pointed out (Chapter IX) how the epigastrula of the mammals (Fig. 67) can be reduced to the original type of the bell-gastrula, which is now preserved by the amphioxus alone (Fig. 35). Like the latter, the human gastrula and that of all other mammals must be regarded as the ontogenetic reproduction of the phylogenetic form that we call the Gastræa, in which the whole body is nothing but a double-walled gastric sac.

We already know from embryology the manner in which the gut develops in the embryo of man and the other mammals. From the gastrula is first formed the spherical embryonic vesicle filled with fluid ( gastrocystis, Fig. 106). In the dorsal wall of this the sole-shaped embryonic shield is developed, and on the under-side of this a shallow groove appears in the middle line, the first trace of the later, secondary alimentary tube. The gut-groove becomes deeper and deeper, and its edges bend towards each other, and finally form a tube.

As we have seen, this simple cylindrical gut-tube is at first completely closed before and behind in man and in the Vertebrates generally (Fig. 148); the permanent openings of the alimentary canal, the mouth and anus, are only formed later on, and from the outer skin. A mouth-pit appears in the skin in front (Fig. 350 hp), and this grows towards the blind fore-end of the cavity of the head-gut ( kd), and at length breaks into it. In the same way a shallow anus-pit is formed in the skin behind, which grows deeper and deeper, advances towards the blind hinder end of the pelvic gut, and at last connects with it. There is at first, both before and behind, a thin partition between the external cutaneous pit and the blind end of the gut—the throat-membrane in front and the anus-membrane behind; these disappear when the connection takes place.

Directly in front of the anus-opening the allantois develops from the hind gut; this is the important embryonic structure that forms into the placenta in the Placentals (including man). In this more advanced form the human alimentary canal (and that of all the other mammals) is a slightly bent, cylindrical tube, with an opening at each end, and two appendages growing from its lower wall: the anterior one is the umbilical vesicle or yelk-sac, and the posterior the allantois or urinary sac (Fig. 195).

The thin wall of this simple alimentary tube and its ventral appendages is found, on microscopic examination, to consist of two strata of cells. The inner stratum, lining the entire cavity, consists of larger and darker cells, and is the gut-gland layer. The outer stratum consists of smaller and lighter cells, and is the gut-fibre layer. The only exception is in the cavities of the mouth and anus, because these originate from the skin. The inner coat of the mouth-cavity is not provided by the gut-gland layer, but by the skin-sense layer; and its muscular substratum is provided, not by the gut-fibre, but the skin-fibre, layer. It is the same with the wall of the small anus-cavity.

If it is asked how these constituent layers of the primitive gut-wall are related to the various tissues and organs that we find afterwards in the fully-developed system, the answer is very simple. It can be put in a single sentence. The epithelium of the gut—that is to say, the internal soft stratum of cells that lines the cavity of the alimentary canal and all its appendages, and is immediately occupied with the processes of nutrition—is formed solely from the gut-gland layer; all other tissues and organs that belong to the alimentary canal and its appendages originate from the gut-fibre layer. From the latter is also developed the whole of the outer envelope of the gut and its appendages; the fibrous connective tissue and the smooth muscles that compose its muscular layer, the cartilages that support it (such as the cartilages of the larynx and the trachea), the blood-vessels and lymph-vessels that absorb the nutritive fluid from the intestines—in a word, all that there is in the alimentary system besides the epithelium of the gut. From the same layer we also get the whole of the mesentery, with all the organs embedded in it—the heart, the large blood-vessels of the body, etc.

Fig. 351—Scales or cutaneous teeth of a shark ( Centrophorus calceus). A three-pointed tooth rises obliquely on each of the quadrangular bony plates that lie in the corium. (From Gegenbaur.)

Let us now leave this original structure of the mammal gut for a moment, in order to compare it with the alimentary canal of the lower Vertebrates, and of those Invertebrates that we have recognised as man’s ancestors. We find, first of all, in the lowest Metazoa, the Gastræads, that the gut remains permanently in the very simple form in which we find it transitorily in the palingenetic gastrula of the other animals; it is thus in the Gastremaria ( Pemmatodiscus), the Physemaria ( Prophysema), the simplest Sponges ( Olynthus), the freshwater Polyps ( Hydra), and the ascula-embryos of many other Cœlenteria (Figs. 233–238). Even in the simplest forms of the Platodes, the Rhabdocœla (Fig. 240), the gut is still a simple straight tube, lined with the entoderm; but with the important difference that in this case its single opening, the primitive mouth ( m), has formed a muscular gullet ( sd) by invagination of the skin.

We have the same simple form in the gut of the lowest Vermalia (Gastrotricha, Fig. 242, Nematodes, Sagitta, etc.). But in these a second important opening of the gut has been formed at the opposite end to the mouth, the anus (Fig. 242 a).

We see a great advance in the structure of the vermalian gut in the remarkable Balanoglossus (Fig. 245), the sole survivor of the Enteropneust class. Here we have the first appearance of the division of the alimentary tube into two sections that characterises the Chordonia. The fore half, the head-gut ( cephalogaster), becomes the organ of respiration (branchial gut, Fig. 245 k); the hind half, the trunk-gut ( truncogaster), alone acts as digestive organ (hepatic gut, d). The differentiation of these two parts of the gut in the Enteropneust is just the same as in all the Tunicates and Vertebrates.

Fig. 352—Gut of a human embryo, one-sixth of an inch long. (From His.)

It is particularly interesting and instructive in this connection to compare the Enteropneusts with the Ascidia and the Amphioxus (Figs. 220, 210)—the remarkable animals that form the connecting link between the Invertebrates and the Vertebrates. In both forms the gut is of substantially the same construction; the anterior section forms the respiratory branchial gut, the posterior the digestive hepatic gut. In both it develops palingenetically from the primitive gut of the gastrula, and in both the hinder end of the medullary tube covers the primitive mouth to such an extent that the remarkable medullary intestinal duct is formed, the passing communication between the neural and intestinal tubes ( canalis neurentericus, Figs. 83, 85 ne). In the vicinity of the closed primitive mouth, possibly in its place, the later anus is developed. In the same way the mouth is a fresh formation in the Amphioxus and the Ascidia. It is the same with the human mouth and that of the Craniotes generally. The secondary formation of the mouth in the Chordonia is probably connected with the development of the gill-clefts which are formed in the gut-wall immediately behind the mouth. In this way the anterior section of the gut is converted into a respiratory organ. I have already pointed out that this modification is distinctive of the Vertebrates and Tunicates. The phylogenetic appearance of the gill-clefts indicates the commencement of a new epoch in the stem-history of the Vertebrates.

In the further ontogenetic development of the alimentary canal in the human embryo the appearance of the gill-clefts is the most important process. At a very early stage the gullet-wall joins with the external body-wall in the head of the human embryo, and this is followed by the formation of four clefts, which lead directly into the gullet from without, on the right and left sides of the neck, behind the mouth. These are the gill or gullet clefts, and the partitions that separate them are the gill or gullet-arches (Fig. 171). These are most interesting embryonic structures. They show us that all the higher Vertebrates reproduce in their earlier stages, in harmony with the biogenetic law, the process that had so important a part in the rise of the whole Chordonia-stem. This process was the differentiation of the gut into two sections—an anterior respiratory section, the branchial gut, that was restricted to breathing, and a posterior digestive section, the hepatic gut. As we find this highly characteristic differentiation of the gut into two different sections in all the Vertebrates and all the Tunicates, we may conclude that it was also found in their common ancestors, the Prochordonia—especially as even the Enteropneusts have it. (Cf. pp. 119, 151, 227, Figs. 210, 220, 245.) It is entirely wanting in all the other Invertebrates.

Fig. 353—Gut of a dog-embryo (shown in Fig. 202, from Bischoff), seen from the ventral side. a gill-arches (four pairs), b rudiments of pharynx and larynx, c lungs, d stomach, f liver, g walls of the open yelk-sac (into which the middle gut opens with a wide aperture), h rectum.
Fig. 354—The same gut seen from the right. a lungs, b stomach, c liver, d yelk-sac, e rectum.)

There is at first only one pair of gill-clefts in the Amphioxus, as in the Ascidia and Enteropneusts; and the Copelata (Fig. 225) have only one pair throughout life. But the number presently increases in the former. In the Craniotes, however, it decreases still further. The Cyclostomes have six to eight pairs (Fig. 247); some of the Selachii six or seven pairs, most of the fishes only four or five pairs. In the embryo of man, and the higher Vertebrates generally, where they make an appearance at an early stage, only three or four pairs are developed. In the fishes they remain throughout life, and form an exit for the water taken in at the mouth (Figs. 249–251). But they are partly lost in the amphibia, and entirely in the higher Vertebrates. In these nothing is left but a relic of the first gill-cleft. This is formed into a part of the organ of hearing; from it are developed the external meatus, the tympanic cavity, and the Eustachian tube. We have already considered these remarkable structures, and need only point here to the interesting fact that our middle and external ear is a modified inheritance from the fishes. The branchial arches also, which separate the clefts, develop into very different parts. In the fishes they remain gill-arches, supporting the respiratory gill-leaves. It is the same with the lowest amphibia, but in the higher amphibia they undergo various modifications; and in the three higher classes of Vertebrates (including man) the hyoid bone and the ossicles of the ear develop from them. (Cf. p. 291.)

From the first gill-arch, from the inner surface of which the muscular tongue proceeds, we get the first structure of the maxillary skeleton—the upper and lower jaws, which surround the mouth and support the teeth. These important parts are wholly wanting in the two lowest classes of Vertebrates, the Acrania and Cyclostoma. They appear first in the earliest Selachii (Figs. 248–251), and have been transmitted from this stem-group of the Gnathostomes to the higher Vertebrates. Hence the original formation of the skeleton of the mouth can be traced to these primitive fishes, from which we have inherited it. The teeth are developed from the skin that clothes the jaws. As the whole mouth cavity originates from the outer integument (Fig. 350), the teeth also must come from it. As a fact, this is found to be the case on microscopic examination of the development and finer structure of the teeth. The scales of the fishes, especially of the shark type (Fig. 351), are in the same position as their teeth in this respect (Fig. 252). The osseous matter of the tooth (dentine) develops from the corium; its enamel covering is a secretion of the epidermis that covers the corium. It is the same with the cutaneous teeth or placoid scales of the Selachii. At first the whole of the mouth was armed with these cutaneous teeth in the Selachii and in the earliest amphibia. Afterwards the formation of them was restricted to the edges of the jaws.

Fig. 355—Median section of the head of a Petromyzon-larva. (From Gegenbaur.) h hypobranchial groove (above it in the gullet we see the internal openings of the seven gill-clefts), v velum, o mouth, c heart, a auditory vesicle, n neural tube, ch chorda.

Hence our human teeth are, in relation to their original source, modified fish-scales. For the same reason we must regard the salivary glands, which open into the mouth, as epidermic glands, as they are formed, not from the glandular layer of the gut like the rest of the alimentary glands, but from the epidermis, from the horny plate of the outer germinal layer. Naturally, in harmony with this evolution of the mouth, the salivary glands belong genetically to one series with the sudoriferous, sebaceous, and mammary glands.

Thus the human alimentary canal is as simple as the primitive gut of the gastrula in its original structure. Later it resembles the gut of the earliest Vermalia (Gastrotricha). It then divides into two sections, a fore or branchial gut and a hind or hepatic gut, like the alimentary canal of the Balanoglossus, the Ascidia, and the Amphioxus. The formation of the jaws and the branchial arches changes it into a real fish-gut ( Selachii). But the branchial gut, the one reminiscence of our fish-ancestors, is afterwards atrophied as such. The parts of it that remain are converted into entirely different structures.

Fig. 356—Transverse section of the head of a Petromyzon-larva. (From Gegenbaur.) Beneath the pharynx ( d) we see the hypobranchial groove; above it the chorda and neural tube. A, B, C stages of constriction.

But, although the anterior section of our alimentary canal thus entirely loses its original character of branchial gut, it retains the physiological character of respiratory gut. We are now astonished to find that the permanent respiratory organ of the higher Vertebrates, the air-breathing lung, is developed from this first part of the alimentary canal. Our lungs, trachea, and larynx are formed from the ventral wall of the branchial gut. The whole of the respiratory apparatus, which occupies the greater part of the pectoral cavity in the adult man, is at first merely a small pair of vesicles or sacs, which grow out of the floor of the head-gut immediately behind the gills (Figs. 354 c, 147 l). These vesicles are found in all the Vertebrates except the two lowest classes, the Acrania and Cyclostomes. In the lower Vertebrates they do not develop into lungs, but into a large air-filled bladder, which occupies a good deal of the body-cavity and has a quite different purport. It serves, not for breathing, but to effect swimming movements up and down, and so is a sort of hydrostatic apparatus—the floating bladder of the fishes ( nectocystis, p. 233). However, the human lungs, and those of all air-breathing Vertebrates, develop from the same simple vesicular appendage of the head-gut that becomes the floating bladder in the fishes.

At first this bladder has no respiratory function, but merely acts as hydrostatic apparatus for the purpose of increasing or lessening the specific gravity of the body. The fishes, which have a fully-developed floating bladder, can press it together, and thus condense the air it contains. The air also escapes sometimes from the alimentary canal, through an air-duct that connects the floating bladder with the pharynx, and is ejected by the mouth. This lessens the size of the bladder, and so the fish becomes heavier and sinks. When it wishes to rise again, the bladder is expanded by relaxing the pressure. In many of the Crossopterygii the wall of the bladder is covered with bony plates, as in the Triassic Undina (Fig. 254).

This hydrostatic apparatus begins in the Dipneusts to change into a respiratory organ; the blood-vessels in the wall of the bladder now no longer merely secrete air themselves, but also take in fresh air through the air-duct. This process reaches its full development in the Amphibia. In these the floating bladder has turned into lungs, and the air-passage into a trachea. The lungs of the Amphibia have been transmitted to the three higher classes of Vertebrates. In the lowest Amphibia the lungs on either side are still very simple transparent sacs with thin walls, as in the common water-salamander, the Triton. It still entirely resembles the floating bladder of the fishes. It is true that the Amphibia have two lungs, right and left. But the floating bladder is also double in many of the fishes (such as the early Ganoids), and divides into right and left halves. On the other hand, the lung is single in Ceratodus (Fig. 257).

Fig. 357—Thoracic and abdominal viscera of a human embryo of twelve weeks. (From Kölliker.) The head is omitted. Ventral and pectoral walls are removed. The greater part of the body-cavity is taken up with the liver, from the middle part of which the cæcum and the vermiform appendix protrude. Above the diaphragm, in the middle, is the conical heart; to the right and left of it are the two small lungs.

In the human embryo and that of all the other Amniotes the lungs develop from the hind part of the ventral wall of the head-gut (Fig. 149). Immediately behind the single structure of the thyroid gland a median groove, the rudiment of the trachea, is detached from the gullet. From its hinder end a couple of vesicles develop—the simple tubular rudiments of the right and left lungs. They afterwards increase considerably in size, fill the greater part of the thoracic cavity, and take the heart between them. Even in the frogs we find that the simple sac has developed into a spongy body of peculiar froth-like tissue. The originally short connection of the pulmonary sacs with the head-gut extends into a long, thin tube. This is the wind-pipe (trachea); it opens into the gullet above, and divides below into two branches which go to the two lungs. In the wall of the trachea circular cartilages develop, and these keep it open. At its upper end, underneath its pharyngeal opening, the larynx is formed—the organ of voice and speech. The larynx is found at various stages of development in the Amphibia, and comparative anatomists are in a position to trace the progressive growth of this important organ from the rudimentary structure of the lower Amphibia up to the elaborate and delicate vocal apparatus that we have in the larynx of man and of the birds.

We must refer here to an interesting rudimentary organ of the respiratory gut, the thyroid gland, the large gland in front of the larynx, that lies below the “Adam’s apple,” and is often especially developed in the male sex. It has a certain function—not yet fully understood—in the nutrition of the body, and arises in the embryo by constriction from the lower wall of the pharynx. In many mining districts the thyroid gland is peculiarly liable to morbid enlargement, and then forms goitre, a growth that hangs at the front of the neck. But it is much more interesting phylogenetically. As Wilhelm Müller, of Jena, has shown, this rudimentary organ is the last relic of the hypobranchial groove, which we considered in a previous chapter, and which runs in the middle line of the gill-crate in the Ascidia and Amphioxus, and conveys food to the stomach. (Cf. p. 184,Fig. 246). We still find it in its original character in the larvæ of the Cyclostomes (Figs. 355, 356).

The second section of the alimentary canal, the trunk or hepatic gut, undergoes not less important modifications among our vertebrate ancestors than the first section. In tracing the further development of this digestive part of the gut, we find that most complex and elaborate organs originate from a very rudimentary original structure. For clearness we may divide the digestive gut into three sections: the fore gut (with œsophagus and stomach), the middle gut (duodenum, with liver, pancreas, jejunum, and ileum, and the hind gut (colon and rectum). Here again we find vesicular growths or appendages of the originally simple gut developing into a variety of organs. Two of these embryonic structures, the yelk-sac and allantois, are already known to us. The two large glands that open into the duodenum, the liver and pancreas, are growths from the middle and most important part of the trunk-gut.

Immediately behind the vesicular rudiments of the lungs comes the section of the alimentary canal that forms the stomach (Figs. 353 d, 354 b). This sac-shaped organ, which is chiefly responsible for the solution and digestion of the food, has not in the lower Vertebrates the great physiological importance and the complex character that it has in the higher. In the Acrania and Cyclostomes and the earlier fishes we can scarcely distinguish a real stomach; it is represented merely by the short piece from the branchial to the hepatic gut. In some of the other fishes also the stomach is only a very simple spindle-shaped enlargement at the beginning of the digestive section of the gut, running straight from front to back in the median plane of the body, underneath the vertebral column. In the mammals its first structure is just as rudimentary as it is permanently in the preceding. But its various parts soon begin to develop. As the left side of the spindle-shaped sac grows much more quickly than the right, and as it turns considerably on its axis at the same time, it soon comes to lie obliquely. The upper end is more to the left, and the lower end more to the right. The foremost end draws up into the longer and narrower canal of the œsophagus. Underneath this on the left the blind sac (fundus) of the stomach bulges out, and thus the later form gradually develops (Figs. 349, 184 e). The original longitudinal axis becomes oblique, sinking below to the left and rising to the right, and approaches nearer and nearer to a transverse position. In the outer layer of the stomach-wall the powerful muscles that accomplish the digestive movements develop from the gut-fibre layer. In the inner layer a number of small glandular tubes are formed from the gut-gland layer; these are the peptic glands that secrete the gastric juice. At the lower end of the gastric sac is developed the valve that separates it from the duodenum (the pylorus, Fig. 349 d).

Underneath the stomach there now develops the disproportionately long stretch of the small intestine. The development of this section is very simple, and consists essentially in an extremely rapid and considerable growth lengthways. It is at first very short, quite straight, and simple. But immediately behind the stomach we find at an early stage a horseshoe-shaped bend and loop of the gut, in connection with the severance of the alimentary canal from the yelk-sac and the development of the first mesentery. The thin delicate membrane that fastens this loop to the ventral side of the vertebral column, and fills the inner bend of the horseshoe formation, is the first rudiment of the mesentery (Fig. 147 g). We find at an early stage a considerable growth of the small intestine; it is thus forced to coil itself in a number of loops. The various sections that we have to distinguish in it are differentiated in a very simple way—the duodenum (next to the stomach), the succeeding long jejunum, and the last section of the small intestine, the ileum.

From the duodenum are developed the two large glands that we have already mentioned—the liver and pancreas. The liver appears first in the shape of two small sacs, that are found to the right and left immediately behind the stomach (Figs. 353 f, 354 c). In many of the lower Vertebrates they remain separate for a long time (in the Myxinoides throughout life), or are only imperfectly joined. In the higher Vertebrates they soon blend more or less completely to form a single large organ. The growth of the liver is very brisk at first. In the human embryo it grows so much in the second month of development that in the third it occupies by far the greater part of the body-cavity (Fig. 357). At first the two halves develop equally; afterwards the left falls far behind the right. In consequence of the unsymmetrical development and turning of the stomach and other abdominal viscera, the whole liver is now pushed to the right side. Although the liver does not afterwards grow so disproportionately, it is comparatively larger in the embryo at the end of pregnancy than in the adult. Its weight relatively to that of the whole body is 1 : 36 in the adult, and 1 : 18 in the embryo. Hence it is very important physiologically during embryonic life; it is chiefly concerned in the formation of blood, not so much in the secretion of bile.

Immediately behind the liver a second large visceral gland develops from the duodenum, the pancreas or sweetbread. It is wanting in most of the lowest classes of Vertebrates, and is first found in the fishes. This organ is also an outgrowth from the gut.

The last section of the alimentary canal, the large intestine, is at first in the embryo a very simple, short, and straight tube, which opens behind by the anus. It remains thus throughout life in the lower Vertebrates. But it grows considerably in the mammals, coils into various folds, and divides into two sections, the first and longer of which is the colon, and the second the rectum. At the beginning of the colon there is a valve (valvula Bauhini) that separates it from the small intestine. Immediately behind this there is a sac-like growth, which enlarges into the cæcum (Fig. 357 v). In the plant-eating mammals this is very large, but it is very small or completely atrophied in the flesh-eaters. In man, and most of the apes, only the first portion of the cæcum is wide; the blind end-part of it is very narrow, and seems later to be merely a useless appendage of the former. This “vermiform appendage” is very interesting as a rudimentary organ. The only significance of it in man is that not infrequently a cherry-stone or some other hard and indigestible matter penetrates into its narrow cavity, and by setting up inflammation and suppuration causes the death of otherwise sound men. Teleology has great difficulty in giving a rational explanation of, and attributing to a beneficent Providence, this dreaded appendicitis. In our plant-eating ancestors this rudimentary organ was much larger and had a useful function.

Finally, we have important appendages of the alimentary tube in the bladder and urethra, which belong to the alimentary system. These urinary organs, acting as reservoir and duct for the urine excreted by the kidneys, originate from the innermost part of the allantoic pedicle. In the Dipneusts and Amphibia, in which the allantoic sac first makes its appearance, it remains within the body-cavity, and functions entirely as bladder. But in all the Amniotes it grows far outside of the body-cavity of the embryo, and forms the large embryonic “primitive bladder,” from which the placenta develops in the higher mammals. This is lost at birth. But the long stalk or pedicle of the allantois remains, and forms with its upper part the middle vesico-umbilical ligament, a rudimentary organ that goes in the shape of a solid string from the vertex of the bladder to the navel. The lowest part of the allantoic pedicle (or the “urachus”) remains hollow, and forms the bladder. At first this opens into the last section of the gut in man as in the lower Vertebrates; thus there is a real cloaca, which takes off both urine and excrements. But among the mammals this cloaca is only permanent in the Monotremes, as it is in all the birds, reptiles, and amphibia. In all the other mammals (marsupials and placentals) a transverse partition is afterwards formed, and this separates the urogenital aperture in front from the anus-opening behind. (Cf. p. 249 and Chapter 29.)

Chapter XXVIII.
EVOLUTION OF THE VASCULAR SYSTEM

The use that we have hitherto made of our biogenetic law will give the reader an idea how far we may trust its guidance in phylogenetic investigation. This differs considerably in the various systems of organs; the reason is that heredity and variability have a very different range in these systems. While some of them faithfully preserve the original palingenetic development inherited from earlier animal ancestors, others show little trace of this rigid heredity; they are rather disposed to follow new and divergent cenogenetic lines of development in consequence of adaptation. The organs of the first kind represent the conservative element in the multicellular state of the human frame, while the latter represent the progressive element. The course of historic development is a result of the correlation of the two tendencies, and they must be carefully distinguished.

There is perhaps no other system of organs in the human body in which this is more necessary than in that of which we are now going to consider the obscure development—the vascular system, or apparatus of circulation. If we were to draw our conclusions as to the original features in our earlier animal ancestors solely from the phenomena of the development of this system in the embryo of man and the other higher Vertebrates, we should be wholly misled. By a number of important embryonic adaptations, the chief of which is the formation of an extensive food-yelk, the original course of the development of the vascular system has been so much falsified and curtailed in the higher Vertebrates that little or nothing now remains in their embryology of some of the principal phylogenetic features. We should be quite unable to explain these if comparative anatomy and ontogeny did not come to our assistance.

The vascular system in man and all the Craniotes is an elaborate apparatus of cavities filled with juices or cell-containing fluids. These “vessels” (vascula) play an important part in the nutrition of the body. They partly conduct the nutritive red blood to the various parts of the body (blood-vessels); partly absorb from the gut the white chyle formed in digestion (chyle-vessels); and partly collect the used-up juices and convey them away from the tissues (lymphatic vessels). With the latter are connected the large cavities of the body, especially the body-cavity, or cœloma. The lymphatic vessels conduct both the colourless lymph and the white chyle into the venous part of the circulation. The lymphatic glands act as producers of new blood-cells, and with them is associated the spleen. The centre of movement for the circulation of the fluids is the heart, a strong muscular sac, which contracts regularly and is equipped with valves like a pump. This constant and steady circulation of the blood makes possible the complex metabolism of the higher animals.

But, however important the vascular system may be to the more advanced and larger and highly-differentiated animals, it is not at all so indispensable an element of animal life as is commonly supposed. The older science of medicine regarded the blood as the real source of life. Even in the still prevalent confused notions of heredity the blood plays the chief part. People speak generally of full blood, half blood, etc., and imagine that the hereditary transmission of certain characters “lies in the blood.” The incorrectness of these ideas is clearly seen from the fact that in the act of generation the blood of the parents is not directly transmitted to the offspring, nor does the embryo possess blood in its early stages. We have already seen that not only the differentiation of the four secondary germinal layers, but also the first structures of the principal organs in the embryo of all the Vertebrates, take place long before there is any trace of the vascular system—the heart and the blood. In accordance with this ontogenetic fact, we must regard the vascular system as one of the latest organs from the phylogenetic point of view; just as we have found the alimentary canal to be one of the earliest. In any case, the vascular system is much later than the alimentary.

Fig. 358—Red blood-cells of various Vertebrates (equally magnified). 1. of man, 2. camel, 3. dove, 4. proteus, 5. water-salamander (Triton), 6. frog, 7. merlin (Cobitis), 8. lamprey (Petromyzon). a surface-view, b edge-view. (From Wagner.)
Fig. 359—Vascular tissues or endothelium (vasalium). A capillary from the mesentery. a vascular cells, b their nuclei.

The important nutritive fluid that circulates as blood and lymph in the elaborate canals of our vascular system is not a clear, simple fluid, but a very complex chemical juice with millions of cells floating in it. These blood-cells are just as important in the complicated life of the higher animal body as the circulation of money is to the commerce of a civilised community. Just as the citizens meet their needs most conveniently by means of a financial circulation, so the various tissue-cells, the microscopic citizens of the multicellular human body, have their food conveyed to them best by the circulating cells in the blood. These blood cells (hæmocytes) are of two kinds in man and all the other Craniotes—red cells (rhodocytes or erythrocytes) and colourless or lymph cells (leucocytes). The red colour of the blood is caused by the great accumulation of the former, the others circulate among them in much smaller quantity. When the colourless cells increase at the expense of the red we get anæmia (or chlorosis).

The lymph-cells (leucocytes), commonly called the “white corpuscles” of the blood, are phylogenetically older and more widely distributed in the animal world than the red. The great majority of the Invertebrates that have acquired an independent vascular system have only colourless lymph-cells in the circulating fluid. There is an exception in the Nemertines (Fig. 358) and some groups of Annelids. When we examine the colourless blood of a cray-fish or a snail (Fig. 358) under a high power of the microscope, we find in each drop numbers of mobile leucocytes, which behave just like independent Amoebæ (Fig. 17). Like these unicellular Protozoa, the colourless blood-cells creep slowly about, their unshapely plasma-body constantly changing its form, and stretching out finger-like processes first in one direction, then another. Like the Amoebæ, they take particles into their cell-body. On account of this feature these amoeboid plastids are called “eating cells” (phagocytes), and on account of their motions “travelling cells” (planocytes). It has been shown by the discoveries of the last few decades that these leucocytes are of the greatest physiological and pathological consequence to the organism. They can absorb either solid or dissolved particles from the wall of the gut, and convey them to the blood in the chyle; they can absorb and remove unusable matter from the tissues. When they pass in large quantities through the fine pores of the capillaries and accumulate at irritated spots, they cause inflammation. They can consume and destroy bacteria, the dreaded vehicles of infectious diseases; but they can also transport these injurious Monera to fresh regions, and so extend the sphere of infection. It is probable that the sensitive and travelling leucocytes of our invertebrate ancestors have powerfully co-operated for millions of years in the phylogenesis of the advancing animal organisation.

Fig. 360—Transverse section of the trunk of a chick-embryo, forty-five hours old. (From Balfour.) A ectoderm (horny-plate), Mc medullary tube, ch chorda, C entoderm (gut-gland layer), Pv primitive segment (episomite), Wd prorenal duct, pp cœloma (secondary body-cavity). So skin-fibre layer, Sp gut-fibre layer, v blood-vessels in latter, ao primitive aortas, containing red blood-cells.

The red blood-cells have a much more restricted sphere of distribution and activity. But they also are very important in connection with certain functions of the craniote-organism, especially the exchange of gases or respiration. The cells of the dark red, carbonised or venous, blood, which have absorbed carbonic acid from the animal tissues, give this off in the respiratory organs; they receive instead of it fresh oxygen, and thus bring about the bright red colour that distinguishes oxydised or arterial blood. The red colouring matter of the blood (hæmoglobin) is regularly distributed in the pores of their protoplasm. The red cells of most of the Vertebrates are elliptical flat disks, and enclose a nucleus of the same shape; they differ a good deal in size (Fig. 358). The mammals are distinguished from the other Vertebrates by the circular form of their biconcave red cells and by the absence of a nucleus (Fig. 1); only a few genera still have the elliptic form inherited from the reptiles (Fig. 2). In the embryos of the mammals the red cells have a nucleus and the power of increasing by cleavage (Fig. 10).

The origin of the blood-cells and vessels in the embryo, and their relation to the germinal layers and tissues, are among the most difficult problems of ontogeny—those obscure questions on which the most divergent opinions are still advanced by the most competent scientists. In general, it is certain that the greater part of the cells that compose the vessels and their contents come from the mesoderm—in fact, from the gut-fibre layer; it was on this account that Baer gave the name of “vascular layer” to this visceral layer of the coeloma. But other important observers say that a part of these cells come from other germinal layers, especially from the gut-gland layer. It seems to be true that blood-cells may be formed from the cells of the entoderm before the development of the mesoderm. If we examine sections of chickens, the earliest and most familiar subjects of embryology, we find at an early stage the “primitive-aortas” we have already described (Fig. 360 ao) in the ventral angle between the episoma (Pv) and hyposoma (Sp). The thin wall of these first vessels of the amniote embryo consists of flat cells (endothelia or vascular epithelia); the fluid within already contains numbers of red blood-cells; both have been developed from the gut-fibre layer. It is the same with the vessels of the germinative area (Fig. 361 v), which lie on the entodermic membrane of the yelk-sac (c). These features are seen still more clearly in the transverse section of the duck-embryo in Fig. 152.In this we see clearly how a number of stellate cells proceed from the “vascular layer” and spread in all directions in the “primary body-cavity”—i.e. in the spaces between the germinal layers. A part of these travelling cells come together and line the wall of the larger spaces, and thus form the first vessels; others enter into the cavity, live in the fluid that fills it, and multiply by cleavage—the first blood-cells.

But, besides these mesodermic cells of the “vascular layer” proper, other travelling cells, of which the origin and purport are still obscure, take part in the formation of blood in the meroblastic Vertebrates (especially fishes). The chief of these are those that Ruckert has most aptly denominated “merocytes.” These “eating yelk-cells” are found in large numbers in the food-yelk of the Selachii, especially in the yelk-wall—the border zone of the germinal disk in which the embryonic vascular net is first developed. The nuclei of the merocytes become ten times as large as the ordinary cell-nucleus, and are distinguished by their strong capacity for taking colour, or their special richness in chromatin. Their protoplasmic body resembles the stellate cells of osseous tissue (astrocytes), and behaves just like a rhizopod (such as Gromia); it sends out numbers of stellate processes all round, which ramify and stretch into the surrounding food-yelk. These variable and very mobile processes, the pseudopodia of the merocytes, serve both for locomotion and for getting food; as in the real rhizopods, they surround the solid particles of food (granules and plates of yelk), and accumulate round their nucleus the food they have received and digested. Hence we may regard them both as eating-cells (phagocytes) and travelling-cells (planocytes). Their lively nucleus divides quickly and often repeatedly, so that a number of new nuclei are formed in a short time; as each fresh nucleus surrounds itself with a mantle of protoplasm, it provides a new cell for the construction of the embryo. Their origin is still much disputed.

Fig. 361—Merocytes of a shark-embryo, rhizopod-like yelk-cells underneath the embryonic cavity (B). (From Ruckert.) z two embryonic cells, k nuclei of the merocytes, which wander about in the yelk and eat small yelk-plates (d), k smaller, more superficial, lighter nuclei, k′ a deeper nucleus, in the act of cleavage, k* chromatin-filled border-nucleus, freed from the surrounding yelk in order to show the numerous pseudopodia of the protoplasmic cell-body.

Half of the twelve stems of the animal world have no blood-vessels. They make their first appearance in the Vermalia. Their earliest source is the primary body-cavity, the simple space between the two primary germinal layers, which is either a relic of the segmentation-cavity, or is a subsequent formation. Amoeboid planocytes, which migrate from the entoderm and reach this fluid-filled primary cavity, live and multiply there, and form the first colourless blood-cells. We find the vascular system in this very simple form to-day in the Bryozoa, Rotatoria, Nematoda, and other lower Vermalia.

The first step in the improvement of this primitive vascular system is the formation of larger canals or blood-conducting tubes. The spaces filled with blood, the relics of the primary body-cavity, receive a special wall. “Blood-vessels” of this kind (in the narrower sense) are found among the higher worms in various forms, sometimes very simple, at other times very complex. The form that was probably the incipient structure of the elaborate vascular system of the Vertebrates (and of the Articulates) is found in two primordial principal vessels—a dorsal vessel in the middle line of the dorsal wall of the gut, and a ventral vessel that runs from front to rear in the middle line of its ventral wall. From the dorsal vessel is evolved the aorta (or principal artery), from the ventral vessel the principal or subintestinal vein. The two vessels are connected in front and behind by a loop that runs round the gut. The blood contained in the two tubes is propelled by their peristaltic contractions.

Fig. 362—Vascular system of an Annelid (Sænuris), foremost section. d dorsal vessel, v ventral vessel, c transverse connection of two (enlarged in shape of heart). The arrows indicate the direction of the flow of blood. (From Gegenbaur.

The earliest Vermalia in which we first find this independent vascular system are the Nemertina (Fig. 244). As a rule, they have three parallel longitudinal vessels connected by loops, a single dorsal vessel above the gut and a pair of lateral vessels to the right and left. In some of the Nemertina the blood is already coloured, and the red colouring matter is real hæmoglobin, connected with elliptical discoid cells, as in the Vertebrates. The further evolution of this rudimentary vascular system can be gathered from the class of the Annelids in which we find it at various stages of development. First, a number of transverse connections are formed between the dorsal and ventral vessels, which pass round the gut ring-wise (Fig. 362). Other vessels grow into the body-wall and ramify in order to convey blood to it. In addition to the two large vessels of the middle plane there are often two lateral vessels, one to the right and one to the left; as, for instance, in the leech. There are four of these parallel longitudinal vessels in the Enteropneusts (Balanoglossus, Fig. 245). In these important Vermalia the foremost section of the gut has already been converted into a gill-crate, and the vascular arches that rise in the wall of this from the ventral to the dorsal vessel have become branchial vessels.

Fig. 363—Head of a fish-embryo, with rudimentary vascular system, from the left. dc Cuvier’s duct (juncture of the anterior and posterior principal veins), sv venous sinus (enlarged end of Cuvier’s duct), a auricle, v ventricle, abr trunk of branchial artery, s gill-clefts (arterial arches between), ad aorta, c carotid artery, n nasal pit. (From Gegenbaur.

We have a further important advance in the Tunicates, which we have recognised as the nearest blood-relatives of our early vertebrate ancestors. Here we find for the first time a real heart—i.e. a central organ of circulation, driving the blood into the vessels by the regular contractions of its muscular wall, it is of a very rudimentary character, a spindle-shaped tube, passing at both ends into a principal vessel (Fig. 221). By its original position behind the gill-crate, on ventral side of the Tunicates (sometimes more, sometimes less, forward), the head shows clearly that it has been formed by the local enlargement of a section of the ventral vessel. We have already noticed the remarkable alternation of the direction of the blood stream, the heart driving it first from one end, then from the other p. 190. This is very instructive, because in most of the worms (even the Enteropneust) the blood in the dorsal vessel travels from back to front, but in the Vertebrates in the opposite direction. As the Ascidia-heart alternates steadily from one direction to the other, it shows us permanently, in a sense, the phylogenetic transition from the earlier forward direction of the dorsal current (in the worms) to the new backward direction (in the Vertebrates).

As the new direction became permanent in the earlier Prochordonia, which gave rise to the Vertebrate stem, the two vessels that proceed from either end of the tubular heart acquired a fixed function. The foremost section of the ventral vessel henceforth always conveys blood from the heart, and so acts as an artery; the hind section of the same vessel brings the blood from the body to the heart, and so becomes a vein. In view of their relation to the two sections of the gut, we may call the latter the intestinal vein and the former the branchial artery. The blood contained in both vessels, and also in the heart, is venous or carbonised blood—i.e. rich in carbonic acid; on the other hand, the blood that passes from the gills into the dorsal vessel is provided with fresh oxygen—arterial or oxydised blood. The finest branches of the arteries and veins pass into each other in the tissues by means of a network of very fine, ventral, hair-like vessels, or capillaries (Fig. 359).

Fig. 364—The five arterial arches of the Craniotes (1–5) in their original disposition. a arterial cone or bulb, a″ aorta-trunk, c carotid artery (foremost continuation of the roots of the aorta). (From Rathke.)
Fig. 365—The five arterial arches of the birds; the lighter parts of the structure disappear; only the shaded parts remain. Letters as in Fig. 364. s subclavian arteries, p pulmonary artery, p′ branches of same, c′ outer carotid, c″ inner carotid. (From Rathke.)
Fig. 366—The five arterial arches of mammals; letters as in Fig. 365. v vertebral artery, b Botall’s duct (open in the embryo, closed afterwards). (From Rathke.)

When we turn from the Tunicates to the closely-related Amphioxus we are astonished at first to find an apparent retrogression in the formation of the vascular system. As we have seen, the Amphioxus has no real heart; its colourless blood is driven along in its vascular system by the principal vessel itself, which contracts regularly in its whole length (cf. Fig. 210). A dorsal vessel that lies above the gut (aorta) receives the arterial blood from the gills and drives it into the body. Returning from here, the venous blood gathers in a ventral vessel under the gut (intestinal vein), and goes back to the gills. A number of branchial vascular arches, which effect respiration and rise in the wall of the branchial gut from belly to back, absorb oxygen from the water and give off carbonic acid; they connect the ventral with the dorsal vessel. As the same section of the ventral vessel, which also forms the heart in the Craniotes, has developed in the Ascidia into a simple tubular heart, we may regard the absence of this in the Amphioxus as a result of degeneration, a return in this case to the earlier form of the vascular system, as we find it in many of the worms. We may assume that the Acrania that really belong to our ancestral series did not share this retrogression, but inherited the one-chambered heart of the Prochordonia, and transmitted it directly to the earliest Craniotes (cf. the ideal Primitive Vertebrate, Prospondylus, Figs. 98–102).

The further phylogenetic evolution of the vascular system is revealed to us by the comparative anatomy of the Craniotes. At the lowest stage of this group, in the Cyclostomes, we find for the first time the differentiation of the vasorium into two sections: a system of blood-vessels proper, which convey the red blood about the body, and a system of lymphatic vessels, which absorb the colourless lymph from the tissues and convey it to the blood. The lymphatics that absorb from the gut and pour into the blood-stream the milky food-fluid formed by digestion are distinguished by the special name of “chyle-vessels.” While the chyle is white on account of its high proportion of fatty particles, the lymph proper is colourless. Both chyle and lymph contain the colourless amœboid cells (leucocytes, Fig. 12) that we also find distributed in the blood as colourless blood-cells (or “white corpuscles”); but the blood also contains a much larger quantity of red cells, and these give its characteristic colour to the blood of the Craniotes (rhodocytes, Fig. 358). The distinction between lymph, chyle, and blood-vessels which is found in all the Craniotes may be regarded as an outcome of division of labour between various sections of our originally simple vascular system. In the Gnathostomes the spleen makes its first appearance, an organ rich in blood, the chief function of which is the extensive formation of new colourless and red cells. It is not found in the Acrania and Cyclostomes, or any of the Invertebrates. It has been transmitted from the earliest fishes to all the Craniotes.

Figs. 367–70—Metamorphosis of the five arterial arches in the human embryo (diagram from Rathke). la arterial cone, 1, 2, 3, 4, 5 first to fifth pair of arteries, ad trunk of aorta, aw roots of aorta. In Fig. 367 only three, in Fig. 368 all five, of the aortic arches are given (the dotted ones only are developed). In Fig. 369 the first two pairs have disappeared again. In Fig. 370 the permanent trunks of the artery are shown; the dotted parts disappear, s subclavian artery, v vertebral, ax axillary, c carotid (c′ outer, c″ inner carotid), p pulmonary.

The heart also, the central organ of circulation in all the Craniotes, shows an advance in structure in the Cyclostomes. The simple, spindle-shaped heart-tube, found in the same form in the embryo of all the Craniotes, is divided into two sections or chambers in the Cyclostomes, and these are separated by a pair of valves. The hind section, the auricle, receives the venous blood from the body and passes it on to the anterior section, the ventricle. From this it is driven through the trunk of the branchial artery (the foremost section of the ventral vessel or principal vein) into the gills.

In the Selachii an arterial cone is developed from the foremost end of the ventricle, as a special division, cut off by valves. It passes into the enlarged base of the trunk of the branchial artery (Fig. 363 abr). On each side 5–7 arteries proceed from it. These rise between the gill-clefts (s) on the gill-arches, surround the gullet, and unite above into a common trunk-aorta, the continuation of which over the gut corresponds to the dorsal vessel of the worms. As the curved arteries on the gill-arches spread into a network of respiratory capillaries, they contain venous blood in their lower part (as arches of the branchial artery) and arterial blood in the upper part (as arches of the aorta). The junctures of the various aortic arches on the right and left are called the roots of the aorta. Of an originally large number of aortic arches there remain at first six, then (owing to degeneration of the fifth arch) only five, pairs; and from these five pairs (Fig. 364) the chief parts of the arterial system develop in all the higher Vertebrates.

The appearance of the lungs and the atmospheric respiration connected therewith, which we first meet in the Dipneusts, is the next important step in vascular evolution. In the Dipneusts the auricle of the heart is divided by an incomplete partition into two halves. Only the right auricle now receives the venous blood from the veins of the body. The left auricle receives the arterial blood from the pulmonary veins. The two auricles have a common opening into the simple ventricle, where the two kinds of blood mix, and are driven through the arterial cone or bulb into the arterial arches. From the last arterial arches the pulmonary arteries arise (Fig. 365 p). These force a part of the mixed blood into the lungs, the other part of it going through the aorta into the body.

Fig. 371—Heart of a rabbit-embryo, from behind. a vitelline veins, b auricles of the heart, c atrium, d ventricle, e arterial bulb, f base of the three pairs of arterial arches. (From Bischoff.)
Fig. 372—Heart of the same embryo (Fig. 371), from the front. v vitelline veins, a auricle, ca auricular canal, l left ventricle, r right ventricle, ta arterial bulb. (From Bischoff.)

From the Dipneusts upwards we now trace a progressive development of the vascular system, which ends finally with the loss of branchial respiration and a complete separation of the two halves of the circulation. In the Amphibia the partition between the two auricles is complete. In their earlier stages, as tadpoles (Fig. 262), they have still the branchial respiration and the circulation of the fishes, and their heart contains venous blood alone. Afterwards the lungs and pulmonary vessels are developed, and henceforth the ventricle of the heart contains mixed blood. In the reptiles the ventricle and its arterial cone begin to divide into two halves by a longitudinal partition, and this partition becomes complete in the higher reptiles and birds on the one hand, and the stem-forms of the mammals on the other. Henceforth, the right half of the heart contains only venous, and the left half only arterial, blood, as we find in all birds and mammals. The right auricle receives its carbonised or venous blood from the veins of the body, and the right ventricle drives it through the pulmonary arteries into the lungs. From here the blood returns, as oxydised or arterial blood, through the pulmonary veins to the left auricle, and is forced by the left ventricle into the arteries of the body. Between the pulmonary arteries and veins is the capillary system of the small or pulmonary circulation. Between the body-arteries and veins is the capillary system of the large or body-circulation. It is only in the two highest classes of Vertebrates—the birds and mammals—that we find a complete division of the circulations. Moreover, this complete separation has been developed quite independently in the two classes, as the dissimilar formation of the aortas shows of itself. In the birds the right half of the fourth arterial arch has become the permanent arch (Fig. 365). In the mammals this has been developed from the left half of the same fourth arch (Fig. 366).

Fig. 373—Heart and head of a dog-embryo, from the front. a fore brain, b eyes, c middle brain, d primitive lower jaw, e primitive upper jaw, f gill-arches, g right auricle, h left auricle, i left ventricle, k right ventricle. (From Bischoff.)
Fig. 374—Heart of the same dog-embryo, from behind. a inosculation of the vitelline veins, b left auricle, c right auricle, d auricle, e auricular canal, f left ventricle, g right ventricle, h arterial bulb. (From Bischoff.)

If we compare the fully-developed arterial system of the various classes of Craniotes, it shows a good deal of variety, yet it always proceeds from the same fundamental type. Its development is just the same in man as in the other mammals; in particular, the modification of the six pairs of arterial arches is the same in both (Figs. 367–370). At first there is only a single pair of arches, which lie on the inner surface of the first pair of gill-arches. Behind this there then develop a second and third pair of arches (lying on the inner side of the second and third gill-arches, Fig. 367). Finally, we get a fourth, fifth, and sixth pair. Of the six primitive arterial arches of the Amniotes three soon pass away (the first, second, and fifth); of the remaining three, the third gives the carotids, the fourth the aortas, and the sixth (number 5 in Figs. 364 and 368) the pulmonary arteries.

Fig. 375—Heart of a human embryo, four weeks old; 1. front view, 2. back view, 3. opened, and upper half of the atrium removed. a′ left auricle, a″ right auricle, v′ left ventricle, v″ right ventricle, ao arterial bulb, c superior vena cava (cd right, cs left), s rudiment of the interventricular wall. (From Kölliker.)
Fig. 376—Heart of a human embryo, six weeks old, front view. r right ventricle, t left ventricle, s furrow between ventricles, ta arterial bulb, af furrow on its surface; to right and left are the two large auricles. (From Ecker.)
Fig. 377—Heart of a human embryo, eight weeks old, back view. a′ left auricle, a″ right auricle, v′ left ventricle, v″ right ventricle, cd right superior vena cava, ci inferior vena cava. (From Kölliker.)

The human heart also develops in just the same way as that of the other mammals (Fig. 378). We have already seen the first rudiments of its embryology, which in the main corresponds to its phylogeny (Figs. 201, 202). We saw that the palingenetic form of the heart is a spindle-shaped thickening of the gut-fibre layer in the ventral wall of the head-gut. The structure is then hollowed out, forms a simple tube, detaches from its place of origin, and henceforth lies freely in the cardiac cavity. Presently the tube bends into the shape of an S, and turns spirally on an imaginary axis in such a way that the hind part comes to lie on the dorsal surface of the fore part. The united vitelline veins open into the posterior end. From the anterior end spring the aortic arches.

Fig. 378—Heart of the adult man, fully developed, front view, natural position. a right auricle (underneath it the right ventricle), b left auricle (under it the left ventricle), C superior vena cava, V pulmonary veins, P pulmonary artery, d Botalli’s duct, A aorta. (From Meyer.)

This first structure of the human heart, enclosing a very simple cavity, corresponds to the tunicate-heart, and is a reproduction of that of the Prochordonia, but it now divides into two, and subsequently into three, compartments; this reminds us for a time of the heart of the Cyclostomes and fishes. The spiral turning and bending of the heart increases, and at the same time two transverse constrictions appear, dividing it externally into three sections (Figs. 371, 372). The foremost section, which is turned towards the ventral side, and from which the aortic arches rise, reproduces the arterial bulb of the Selachii. The middle section is a simple ventricle, and the hindmost, the section turned towards the dorsal side, into which the vitelline veins inosculate, is a simple auricle (or atrium). The latter forms, like the simple atrium of the fish-heart, a pair of lateral dilatations, the auricles (Fig. 371 b); and the constriction between the atrium and ventricle is called the auricular canal (Fig. 372 ca). The heart of the human embryo is now a complete fish-heart.

In perfect harmony with its phylogeny, the embryonic development of the human heart shows a gradual transition from the fish-heart, through the amphibian and reptile, to the mammal form, The most important point in the transition is the formation of a longitudinal partition—incomplete at first, but afterwards complete—which separates all three divisions of the heart into right (venous) and left (arterial) halves (cf. Figs. 373–378). The atrium is separated into a right and left half, each of which absorbs the corresponding auricle; into the right auricle open the body-veins (upper and lower vena cava, Figs. 375 c, 377 c); the left auricle receives the pulmonary veins. In the same way a superficial interventricular furrow is soon seen in the ventricle (Fig. 376 s). This is the external sign of the internal partition by which the ventricle is divided into two—a right venous and left arterial ventricle. Finally a longitudinal partition is formed in the third section of the primitive fish-like heart, the arterial bulb, externally indicated by a longitudinal furrow (Fig. 376 af). The cavity of the bulb is divided into two lateral halves, the pulmonary-artery bulb, that opens into the right ventricle, and the aorta-bulb, that opens into the left ventricle. When all the partitions are complete, the small (pulmonary) circulation is distinguished from the large (body) circulation; the motive centre of the former is the right half, and that of the latter the left half, of the heart.

Fig. 379—Transverse section of the back of the head of a chick-embryo, forty hours old. (From Kölliker.) m medulla oblongata, ph pharyngeal cavity (head-gut), h horny plate, h′ thicker part of it, from which the auscultory pits afterwards develop, hp skin-fibre plate, hh cervical cavity (head-cœlom or cardiocœl), hzp cardiac plate (the outermost mesodermic wall of the heart), connected by the ventral mesocardium (uhg) with the gut-fibre layer or visceral cœlom-layer (dfp*prime;), Ent entoderm, ihh inner (entodermic?) wall of the heart; the two endothelial cardiac tubes are still separated by the cenogenetic septum (s) of the Amniotes, g vessels.

The heart of all the Vertebrates belongs originally to the hyposoma of the head, and we accordingly find it in the embryo of man and all the other Amniotes right in front on the under-side of the head; just as in the fishes it remains permanently in front of the gullet. It afterwards descends into the trunk, with the advance in the development of the neck and breast, and at last reaches the breast, between the two lungs. At first it lies symmetrically in the middle plane of the body, so that its long axis corresponds with that of the body. In most of the mammals it remains permanently in this position. But in the apes the axis begins to be oblique, and the apex of the heart to move towards the left side. The displacement is greatest in the anthropoid apes—chimpanzee, gorilla, and orang—which resemble man in this.

As the heart of all Vertebrates is originally, in the light of phylogeny, only a local enlargement of the middle principal vein, it is in perfect accord with the biogenetic law that its first structure in the embryo is a simple spindle-shaped tube in the ventral wall of the head-gut. A thin membrane, standing vertically in the middle plane, the mesocardium, connects the ventral wall of the head-gut with the lower head-wall. As the cardiac tube extends and detaches from the gut-wall, it divides the mesocardium into an upper (dorsal) and lower (ventral) plate (usually called the mesocardium anterius and posterius in man, Fig. 379 uhg). The mesocardium divides two lateral cavities, Remak’s “neck-cavities” (Fig. 379 hh). These cavities afterwards join and form the simple pericardial cavity, and are therefore called by Kölliker the “primitive pericardial cavities.”

The double cervical cavity of the Amniotes is very interesting, both from the anatomical and the evolutionary point of view; it corresponds to a part of the hyposomites of the head of the lower Vertebrates—that part of the ventral cœlom-pouches which comes next to Van Wijhe’s “visceral cavities” below. Each of the cavities still communicates freely behind with the two cœlom-pouches of the trunk; and, just as these afterwards coalesce into a simple body-cavity (the ventral mesentery disappearing), we find the same thing happening in the head. This simple primary pericardial cavity has been well called by Gegenbaur the “head-cœloma,” and by Hertwig the “pericardial breast-cavity.” As it now encloses the heart, it may also be called cardiocœl.

Fig. 380—Frontal section of a human embryo, one-twelfth of an inch long in the neck; “invented” by Wilhelm His. Seen from ventral side. mb mouth-fissure, surrounded by the branchial processes, ab bulbus of aorta, hm middle part of ventricle, hl left lateral part of same, ho auricle, d diaphragm, vc superior vena cava, vu umbilical vein, vo vitelline space, lb liver, lg hepatic duct.

The cardiocœl, or head-cœlom, is often disproportionately large in the Amniotes, the simple cardiac tube growing considerably and lying in several folds. This causes the ventral wall of the amniote embryo, between the head and the navel, to be pushed outwards as in rupture (cf. Fig. 180 h). A transverse fold of the ventral wall, which receives all the vein-trunks that open into the heart, grows up from below between the pericardium and the stomach, and forms a transverse partition, which is the first structure of the primary diaphragm (Fig. 380 d). This important muscular partition, which completely separates the thoracic and abdominal cavities in the mammals alone, is still very imperfect here; the two cavities still communicate for a time by two narrow canals. These canals, which belong to the dorsal part of the head-cœlom, and which we may call briefly pleural ducts, receive the two pulmonary sacs, which develop from the hind end of the ventral wall of the head-gut; they thus become the two pleural cavities.

The diaphragm makes its first appearance in the class of the Amphibia (in the salamanders) as an insignificant muscular transverse fold of the ventral wall, which rises from the fore end of the transverse abdominal muscle, and grows between the pericardium and the liver. In the reptiles (tortoises and crocodiles) a later dorsal part is joined to this earlier ventral part of the rudimentary diaphragm, a pair of subvertebral muscles rising from the vertebral column and being added as “columns” to the transverse partition. But it was probably in the Permian sauro-mammals that the two originally separate parts were united, and the diaphragm became a complete partition between the thoracic and abdominal cavities in the mammals; as it considerably enlarges the chest-cavity when it contracts, it becomes an important respiratory muscle. The ontogeny of the diaphragm in man and the other mammals reproduces this phylogenetic process to-day, in accordance with the biogenetic law; in all the mammals the diaphragm is formed by the secondary conjunction of the two originally separate structures, the earlier ventral part and the later dorsal part.

Sometimes the blending of the two diaphragmatic structures, and consequently the severance of the one pleural duct from the abdominal cavity, is not completed in man. This leads to a diaphragmatic rupture (hernia diaphragmatica). The two cavities then remain in communication by an open pleural duct, and loops of the intestine may penetrate by this “rupture opening” into the chest-cavity. This is one of those fatal mis-growths that show the great part that blind chance has in organic development.

Fig. 381—Transverse section of the head of a chick-embryo, thirty-six hours old. Underneath the medullary tube the two primitive aortas (pa) can be seen in the head-plates (s) at each side of the chorda. Underneath the gullet (d) we see the aorta-end of the heart (ae), hh cervical cavity or head cœlom, hk top of heart, ks head-sheath, amniotic fold, h horny plate. (From Remak.

Thus the thoracic cavity of the mammals, with its important contents, the heart and lungs, belongs originally to the head-part of the vertebrate body, and its inclusion in the trunk is secondary. This instructive and very interesting fact is entirely proved by the concordant evidence of comparative anatomy and ontogeny. The lungs are outgrowths of the head-gut; the heart develops from its inner wall. The pleural sacs that enclose the lungs are dorsal parts of the head-cœlom, originating from the pleuroducts; the pericardium in which the heart afterwards lies is also double originally, being formed from ventral halves of the head-cœlom, which only combine at a later stage. When the lung of the air-breathing Vertebrates issues from the head-cavity and enters the trunk-cavity, it follows the example of the floating bladder of the fishes, which also originates from the pharyngeal wall in the shape of a small pouch-like out-growth, but soon grows so large that, in order to find room, it has to pass far behind into the trunk-cavity. To put it more precisely, the lung of the quadrupeds retains this hereditary growth-process of the fishes; for the hydrostatic floating bladder of the latter is the air-filled organ from which the air-breathing organ of the former has been evolved.

Fig. 382—Transverse section of the cardiac region of the same chick-embryo (behind the preceding). In the cervical cavity (hh) the heart (h) is still connected by a mesocard (hg) with the gut-fibre layer (pf). d gut-gland layer, up provertebral plates, jb rudimentary auditory vesicle in the horny plate, hp first rise of the amniotic fold. (From Remak.)

There is an interesting cenogenetic phenomenon in the formation of the heart of the higher Vertebrates that deserves special notice. In its earliest form the heart is double, as recent observation has shown, in all the Amniotes, and the simple spindle-shaped cardiac tube, which we took as our starting-point, is only formed at a later stage, when the two lateral tubes move backwards, touch each other, and at last combine in the middle line. In man, as in the rabbit, the two embryonic hearts are still far apart at the stage when there are already eight primitive segments (Fig. 134 h). So also the two cœlom-pouches of the head in which they lie are still separated by a broad space. It is not until the permanent body of the embryo develops and detaches from the embryonic vesicle that the separate lateral structures join together, and finally combine in the middle line. As the median partition between the right and left cardiocœl disappears, the two cervical cavities freely communicate (Fig. 381), and form, on the ventral side of the amniote head, a horseshoe-shaped arch, the points of which advance backwards into the pleuro-ducts or pleural cavities, and from there into the two peritoneal sacs of the trunk. But even after the conjunction of the cervical cavities (Fig. 381) the two cardiac tubes remain separate at first; and even after they have united a delicate partition in the middle of the simple endothelial tube (Figs. 379 s, 382 h) indicates the original separation. This cenogenetic “primary cardiac septum” presently disappears, and has no relation to the subsequent permanent partition between the halves of the heart, which, as a heritage from the reptiles, has a great palingenetic importance.

Thorough opponents of the biogenetic law have laid great stress on these and similar cenogenetic phenomena, and endeavoured to urge them as striking disproofs of the law. As in every other instance, careful, discriminating, comparative-morphological examination converts these supposed disproofs of evolution into strong arguments in its favour. In his excellent work, On the structure of the Heart in the Amphibia (1886), Carl Rabl has shown how easily these curious cenogenetic facts can be explained by the secondary adaptation of the embryonic structure to the great extension of the food-yelk.

The embryology of all the other parts of the vascular system also gives us abundant and valuable data for the purposes of phylogeny. But as one needs a thorough knowledge of the intricate structure of the whole vascular system in man and the other Vertebrates in order to follow this with profit, we cannot go into it further here. Moreover, many important features in the ontogeny of the vascular system are still very obscure and controverted. The characters of the embryonic circulation of the Amniotes, which we have previously considered (Chapter XV), are late acquisitions and entirely cenogenetic. (Cf. pp. 170–171; Figs. 198–202.)

Chapter XXIX.
EVOLUTION OF THE SEXUAL ORGANS

If we measure the importance of the systems of organs in the animal frame according to the richness and variety of their phenomena and the physiological interest that this implies, we must regard as one of the principal and most interesting systems the one which we are now going to examine—the system of the reproductive organs. Just as nutrition is the first and most urgent condition for the self-maintenance of the individual organism, so reproduction alone secures the maintenance of the species—or, rather, the maintenance of the long series of generations which the totality of the organic stem represents in their genealogical connection. No individual organism has the prerogative of immortality. To each is allotted only a brief span of personal development, an evanescent moment in the million-year course of the history of life.

Hence, reproduction and the correlative phenomenon, heredity, have long been regarded, together with nutrition, as the most important and fundamental function of living things, and it has been attempted to distinguish them from “lifeless bodies” on this very score. As a matter of fact, this division is not so profound and thorough as it seems to be, and is generally supposed to be. If we examine carefully the nature of the reproductive process, we soon see that it can be reduced to a general property that is found in inorganic as well as organic bodies—growth. Reproduction is a nutrition and growth of the organism beyond the individual limit, which raises a part of it into the whole. This is most clearly seen when we study it in the simplest and lowest organisms, especially the Monera (Figs. 226–228) and the unicellular Amœbæ (Fig. 17). There the simple individual is a single plastid. As soon as it has reached a certain limit of size by continuous feeding and normal growth, it cannot pass it, but divides, by simple cleavage, into two equal halves. Each of these halves then continues its independent life, and grows on until it in turn reaches the limit of growth, and divides. In each of these acts of self-cleavage two new centres of attraction are formed for the particles of bodies, the foundations of the two new-formed individuals. There is no such thing as immortality even in these unicellulars. The individual as such is annihilated in the act of cleavage (cf. p. 48).

In many other Protozoa reproduction takes place not by cleavage, but by budding (gemmation). In this case the growth that determines reproduction is not total (as in segmentation), but partial. Hence in gemmation also we may oppose the local growth-product, that becomes a new individual in the bud, as a child-organism to the parent-organism from which it is formed. The latter is older and larger than the former. In cleavage the two products are equal in age and morphological value. Next to gemmation we have, as other forms of asexual reproduction, the forming of embryonic buds and the forming of embryonic cells. But the latter leads us at once to sexual generation, the distinctive feature of which is the separation of the sexes. I have dealt fully with these various types of reproduction in my History of Creation (chap. viii) and my Wonders of Life (chap. xi).

The earliest ancestors of man and the higher animals had no faculty of sexual reproduction, but multiplied solely by asexual means—cleavage, gemmation, or the formation of embryonic buds or cells, as many Protozoa still do. The differentiation of the sexes came at a later stage. We see this most plainly in the Protists, in which the union of two individuals precedes the continuous cleavage of the unicellular organism (transitory conjugation and permanent copulation of the Infusoria). We may say that in this case the growth (the condition of reproduction) is attained by the coalescence of two full-grown cells into a single, disproportionately large individual. At the same time, the mixture of the two plastids causes a rejuvenation of the plasm. At first the copulating cells are quite homogeneous; but natural selection soon brings about a certain contrast between them—larger female cells (macrospores) and smaller male cells (microspores). It must be a great advantage in the struggle for life for the new individual to have inherited different qualities from the two cellular parents. The further advance of this contrast between the generating cells led to sexual differentiation. One cell became the female ovum (macrogonidion), and the other the male sperm-cell (microgonidion).

The simplest forms of sexual reproduction among the living Metazoa are seen in the Gastræads p. 233, the lower sponges, the common fresh-water polyp (Hydra), and other Cœlenteria of the lowest rank. Prophysema (Fig. 234), Olynthus (Fig. 238), Hydra, etc., have very simple tubular bodies, the thin wall of which consists (as in the original gastrula) only of the two primary germinal layers. As soon as the body reaches sexual maturity, a number of the cells in its wall become female ova, and others male sperm-cells: the former become very large, as they accumulate a considerable quantity of yelk-granules in their protoplasm (Fig. 235 e); the latter are very small on account of their repeated cleavage, and change into mobile cone-shaped spermatozoa (Fig. 20). Both kinds of cells detach from their source of origin, the primary germinal layers, fall either into the surrounding water or into the cavity of the gut, and unite there by fusing together. This is the momentous process of fecundation, which we have examined in Chapter VII (cf. Figs. 23–29).

From these simplest forms of sexual propagation, as we can observe them to-day in the lowest Zoophytes, the Gastræads, Sponges, and Polyps, we gather most important data. In the first place, we learn that, properly speaking, nothing is required for sexual reproduction except the fusion or coalescence of two different cells—a female ovum and male sperm-cell. All other features, and all the very complex phenomena that accompany the sexual act in the higher animals, are of a subordinate and secondary character, and are later additions to this simple, primary process of copulation and fecundation. But if we bear in mind how extremely important a part this relation of the two sexes plays in the whole of organic nature, in the life of plants, of animals, and of man; how the mutual attraction of the sexes, love, is the mainspring of the most remarkable processes—in fact, one of the chief mechanical causes of the highest development of life—we cannot too greatly emphasise this tracing of love to its source, the attractive force of two erotic cells.

Throughout the whole of living nature the greatest effects proceed from this very small cause. Consider the part that the flowers, the sexual organs of the flowering plants, play in nature; or the exuberance of wonderful phenomena that sexual selection produces in animal life; or the momentous influence of love in the life of man. In every case the fusion of two cells is the sole original motive power; in every case this invisible process profoundly affects the development of the most varied structures. We may say, indeed, that no other organic process can be compared to it for a moment in comprehensiveness and intensity of action. Are not the Semitic myth of Adam and Eve, the old Greek legend of Paris and Helena, and so many other famous traditions, only the poetic expression of the vast influence that love and sexual selection have exercised over the course of history ever since the differentiation of the sexes? All the other passions that agitate the heart of man are far outstripped in their joint influence by this sense-inflaming and mind-benumbing Eros. On the one hand, we look to love with gratitude as the source of the greatest artistic achievements—the noblest creations of poetry, plastic art, and music; we see in it the chief factor in the moral advance of humanity, the foundation of family life, and therefore of social advance. On the other hand, we dread it as the devouring flame that brings destruction on so many, and has caused more misery, vice, and crime than all the other evils of human life put together. So wonderful is love and so momentous its influence on the life of the soul, or on the different functions of the medullary tube, that here more than anywhere else the “supernatural” result seems to mock any attempt at natural explanation. Yet comparative evolution leads us clearly and indubitably to the first source of love—the affinity of two different erotic cells, the sperm-cell and ovum.[^[34]]

[34] The sensual perception (probably related to smell) of the two copulating sex-cells, which causes their mutual attraction, is a little understood, but very interesting, chemical function of the cell-soul (cf. p. 58 and The Riddle of the Universe, chap. ix.)

The lowest Metazoa throw light on this very simple origin of the intricate phenomena of reproduction, and they also teach us that the earliest sexual form was hermaphrodism, and that the separation of the sexes (by division of labour) is a secondary and later phenomenon. Hermaphrodism predominates in the most varied groups of the lower animals; each sexually-mature individual, each person, contains female and male sexual cells, and is therefore able to fertilise itself and reproduce. Thus we find ova and sperm-cells in the same individual, not only in the lowest Zoophytes (Gastræads, Sponges, and many Polyps), but also in many worms (leeches and earthworms), many of the snails (the common garden and vineyard snails), all the Tunicates, and many other invertebrate animals. All man’s earlier invertebrate ancestors, from the Gastræads up to the Prochordonia, were hermaphrodites; possibly even the earliest Acrania. We have an instructive proof of this in the remarkable circumstance that many genera of fishes are still hermaphrodites, and that it is occasionally found in the higher Vertebrates of all classes (as atavism). We may conclude from this that gonochorism (separation of the sexes) was a later stage in our development. At first, male and female individuals differ only in the possession of one or other kind of gonads; in other respects they were identical, as we still find in the Amphioxus and the Cyclostomes. Afterwards, accessory organs (ducts, etc.) are associated with the primary sexual glands; and much later again sexual selection has given rise to the secondary sexual characters—those differences between the sexes which do not affect the sexual organs themselves, but other parts of the body (such as the man’s beard or the woman’s breast).

The third important fact that we learn from the lower Zoophytes relates to the earliest origin of the two kinds of sexual cells. As in the Gastræads (the lowest sponges and hydroids), in which we find the first beginnings of sexual differentiation, the whole body consists merely of the two primary germinal layers, it follows that the sexual cells also must have proceeded from the cells of these primary layers, either the inner or outer, or from both. This simple fact is extremely important, because the first trace of the ova as well as the spermatozoa is found in the middle germinal layer or mesoderm in the higher animals, especially the Vertebrates. This arrangement is a later development from the preceding (in connection with the secondary formation of the mesoderm).

If we trace the phylogeny of the sexual organs in our earliest Metazoa ancestors, as the comparative anatomy and ontogeny of the lowest Cœlenteria (Cnidaria, Platodaria) exhibit it to us, we find that the first step in advance is the localisation or concentration of the two kinds of sexual cells scattered in the epithelium into definite groups. In the Sponges and lowest Hydropolyps isolated cells are detached from the cell-strata of the two primary germinal layers, and become free sexual cells; but in the Cnidaria and Platodes we find these associated in groups which we call sexual glands (gonads). We can now for the first time speak of sexual organs in the morphological sense. The female germinative glands, which in this simplest form are merely groups of homogeneous cells, are the ovaries (Fig. 241 c). The male germinative glands, which also in their first form consist of a cluster of sperm-cells, are the testicles (Fig. 241 h). In the medusæ, which descend, both ontogenetically and phylogenetically, from the more simply organised Polyps, we find these simple sexual glands sometimes as gastric pouches, sometimes as outgrowths of the radial canals that proceed from the stomach. Particularly interesting in connection with the question of the first origin of the gonads are the lowest forms of the Platodes, the Cryptocœla that have of late been separated as a special class (Platodaria) from the Turbellaria proper (Fig. 239). In these very primitive Platodes the two pairs of sexual glands are merely two pairs of rows of differentiated cells in the entodermic wall of the primitive gut—two median ovaries (o) within, and two lateral spermaries (s) without. The mature sexual cells are ejected by the posterior outlets; the female (f) lies in front of the male (m).

Fig. 383—Embryos of Sagitta, in three earlier stages of development. (From Hertwig.) A gastrula, B cœlomula with open primitive mouth, C the same primitive mouth closed, ua primitive gut, bl primitive mouth, g progonidia (hermaphroditic primitive sexual cells), cs cœlom-pouches, pm parietal layer, vm visceral layer of same, d permanent gut (enteron), st mouth-pit (stomodæum).

In the great majority of the Bilateria or Cœlomaria it is the mesoderm from which the gonads develop. Probably the first traces of them are the two large cells that appear at the edge of the primitive mouth (right and left), as a rule during gastrulation or immediately afterwards—the important promesoblasts, or “polar cells of the mesoderm,” or “primitive cells of the middle germinal layer” (p. 194). In the real Enterocœla, in which the mesoderm appears from the first in the shape of a couple of cœlom-pouches, these are very probably the original gonads (p. 194). This is seen very clearly in the arrow-worm (Sagitta). In the gastrula of Sagitta (Fig. 383 A) we find at an early stage a couple of entodermic cells of an unusual size (g) at the base of the primitive gut (ud). These primitive sexual cells (progonidia) are symmetrically placed to the right and left of the middle plane, like the two promesoblasts of the bilateral gastrula of the Amphioxus (Fig. 38 p). A little outwards from them the two cœlom pouches (B, cs) are developed out of the primitive gut, and each progonidion divides into a male and a female sexual cell (B, g). The two male cells (at first rather the larger) lie close together within, and are the parent-cells of the testicles (prospermaria). The two female cells lie outwards from these, and are the parent-cells of the ovary (protovaria). Afterwards, when the cœlom-pouches have detached from the permanent gut (C, d) and the primitive mouth (A, bl) is closed, the female cells advance towards the mouth (C, st), and the male towards the rear. The foremost pair of ovaries are then separated by a transverse partition from the hind pair. Thus the first structures of the sexual glands of the Sagitta are a couple of hermaphroditic entodermic cells; each of these divides into a male and a female cell; and these four cells are the parent-cells of the four sexual glands. Probably the two promesoblasts of the Amphioxus-gastrula (Fig. 38) are also hermaphroditic primitive sexual cells in the same sense, inherited by this earliest vertebrate from its ancient bilateral gastræad ancestors.

Fig. 384—A, Part of the kidneys of Bdellostoma. a prorenal duct (nephroductus), b segmental or primitive urinary canals (pronephridia), c renal or Malpighian capsules. B Portion of same, highly magnified. c renal capsules with the glomerulus, d afferent artery, e efferent artery. From Johannes Müller (Myxinoides).

The sexually-mature Amphioxus is not hermaphroditic, as its nearest invertebrate relatives, the Tunicates, are, and as the long-extinct pre-Silurian Primitive Vertebrate (Prospondylus, Figs. 98–102) probably was. The actual lancelet has gonochoristic structures of a very interesting kind. As we saw in the anatomy of the Amphioxus, we find the ovaries of the female and the spermaries of the male in the shape of twenty to thirty pairs of elliptical or roundish four-cornered sacs, which lie on either side of the gut on the parietal surface of the respiratory pore (Fig. 219 g). According to the important discovery of Rückert (1888), the sexual glands of the earliest fishes, the Selachii, are similarly arranged. They only unite afterwards to form a pair of simple gonads. These have been transmitted by heredity to all the rest of the Craniotes. In every case they lie originally on each side of the mesentery, underneath the chorda, at the bottom of the body-cavity. The first traces of them are found in the cœlom-epithelium, at the spot where the skin-fibre layer and gut-fibre layer meet in the middle of the mesenteric plate (Fig. 93 mp). At this point we observe at an early stage in all craniote embryos a small string-like cluster of cells, which we may call, with Waldeyer, the “germ epithelium,” or (in harmony with the other plate-shaped rudimentary organs) the sexual plate (Fig. 173 g). This germinal or sexual plate is found in the fifth week in the human embryo, in the shape of a couple of long whitish streaks, on the inner side of the primitive kidneys (Fig. 183 t). The cells of this sexual plate are distinguished by their cylindrical form and chemical composition from the rest of the cœlom-cells; they have a different purport from the flat cells which line the rest of the body-cavity. As the germ epithelium of the sexual plate becomes thicker, and supporting tissue grows into it from the mesoderm, it becomes a rudimentary sexual gland. This ventral gonad then develops into the ovary in the female Craniotes, and the testicles in the male.

In the formation of the gonidia or erotic sexual cells and their conjunction at fecundation we have the sole essential features of sexual reproduction; but in the great majority of animals we find other organs taking part in it. The chief of these secondary sexual organs are the gonoducts, which serve to convey the mature sexual cells out of the body, and the copulative organs, which bring the fecundating male sperm into touch with the ovum-bearing female. The latter organs are, as a rule, only found in the higher animals, and are much less widely distributed than the gonoducts. But these also are secondary formations, and are wanting in many animals of the lower groups.

In the lower animals the mature sexual cells are generally ejected directly from the body. Sometimes they pass out immediately through the skin (Hydra and many hydroids); sometimes they fall into the gastric cavity, and are evacuated by the mouth (gastræads, sponges, many medusæ, and corals); sometimes they fall into the body-cavity, and are ejected by a special pore (porus genitalis) in the ventral wall. The latter procedure is found in many of the worms, and also in the lowest Vertebrates. Amphioxus has the peculiar feature that the mature sexual products fall first into the mantle-cavity; from there they are either evacuated by the respiratory pore, or else they pass through the gill-clefts into the branchial gut, and so out by the mouth (p. 185). In the Cyclostomes they fall into the body-cavity, and are ejected by a genital pore in its wall; so also in some of the fishes. From these we gather the features of our earlier ancestors in this respect. On the other hand, in all the higher and most of the lower Vertebrates (and most of the higher Invertebrates) we find in both sexes special tubular passages of the sexual gland, which are called “gonoducts.” In the female they conduct the ova from the ovary, and so are called “oviducts,” or “Fallopian tubes.” In the male they convey the spermatozoa from the testicles, and are called “spermaducts,” or vasa deferentia.

Fig. 385—Transverse section of the embryonic shield of a chick, forty-two hours old. (From Kölliker.) mr medullary tube, ch chorda, h horny plate (skin-sense layer), ung nephroduct, vw episomites (dorsal primitive segments), hp skin-fibre layer (parietal layer of the hyposomites), dfp gut-fibre layer (visceral layer of hyposomites), ao aorta, g vessels. (Cf. transverse section of duck-embryo, Fig. 152.)

The original and genetic relation of these two kinds of ducts is just the same in man as in the rest of the higher Vertebrates, and quite different from what we find in most of the Invertebrates. In the latter, as a rule, the gonoducts develop directly from the embryonic glands or from the outer skin; but in the Vertebrates an independent organic system is employed to convey the sexual products, and this had originally a totally different function—namely, the system of urinary organs. These organs have primarily the sole duty of removing unusable matter from the body in a fluid form. Their liquid excretory product, the urine, is either evacuated directly through the skin or through the last section of the gut. It is only at a later stage that the tubular urinary passages also convey the sexual products from the body. In this way they become “urogenital ducts.” This remarkable secondary conjunction of the urinary and sexual organs into a common urogenital system is very characteristic of the Gnathostomes, the six higher classes of Vertebrates. It is wanting in the lower classes. In order to appreciate it fully, we must give a comparative glance at the structure of the urinary organs.

The renal or urinary system is one of the oldest and most important systems of organs in the differentiated animal body, as I have pointed out on several previous occasions (cf. Chapter XVII). We find it not only in the higher stems, but also very generally distributed in the earlier group of the Vermalia. Here we meet it in the lowest worms, the Rotatoria (Gastrotricha, Fig. 242), and in the instructive stem of the Platodes. It consists of a pair of simple or branching canals, which are lined with one layer of cells, absorb unusable juices from the tissue, and eject them by an outlet in the outer skin (Fig. 240 nm). Not only the free-living Turbellaria, but also the parasitic Suctoria, and even the still more degenerate tapeworms, which have lost their alimentary canal in consequence of their parasitic life, are equipped with these renal canals or nephridia. In the first embryonic structure they are merely a pair of simple cutaneous glands, or depressions in the ectoderm. They are generally described as excretory organs in the worms, but formerly often as “water vessels.” They may be conceived as largely-developed tubular cutaneous glands, formed by invagination of the cutaneous layer. According to another view, they owe their origin to a later rupture of the body-cavity outwards. In most of the Vermalia each nephridium has an inner opening (with cilia) into the body-cavity and an outer one on the epidermis.

Fig. 386—Rudimentary primitive kidneys of a dog-embryo. The hind end of the embryonic body is seen from the ventral side and covered with the visceral layer of the yelk-sac, which is torn away and folded down in front in order to show the nephroducts with the primitive urinary canals (a). b primitive vertebræ, c spinal cord, d entrance into the pelvic-gut cavity. (From Bischoff.)

Fig. 387—Primitive kidneys of a human embryo. u the urinary canals of the primitive kidneys, w Wolffian duct, w′ uppermost end of the same (Morgagni’s hydatid), m Mullerian duct. m′ uppermost end of same (Fallopian hydatid), g gonad (sexual gland). (From Kobelt.)

In these lowest, unsegmented worms, and in the unsegmented Molluscs, there is only one pair of renal canals. They are more numerous in the higher Articulates. In the Annelids, the body of which is composed of a large number of joints, there is a pair of these pronephridia in each segment (hence they are called segmental canals or organs). Even here they are still simple tubes; on account of their coiled or looped form they are often called “looped canals.” In most of the Annelids, and many of the Vermalia, we can distinguish three sections in the nephridium—an outer muscular duct, a glandular middle part, and an inner part that opens by a ciliated funnel into the body-cavity. This opening is furnished with whirling cilia, and can, therefore, take up the juices to be excreted directly from the body-cavity and convey them from the body. But in these worms the sexual cells, which develop in very primitive form on the inner surface of the body-cavity, also fall into it when mature, and are sucked up by the funnel-shaped inner ciliated openings of the renal canals, and ejected with the urine. Thus the urine-forming looped canals, or pronephridia, serve as oviducts in the female Annelids and as spermaducts in the male.

The renal system of the Vertebrates is similar to, yet materially different from, these segmental canals of the Annelids. The peculiar development of it and its relations to the sexual organs are among the most difficult problems in the morphology of our stem. If we examine briefly the vertebrate renal system from the phylogenetic point of view, as confirmed by recent discoveries, we may distinguish three forms of it: (1) Fore-kidneys or head-kidneys (pronephros); (2) primitive or middle kidneys (mesonephros); (3) permanent kidneys (metanephros). These three systems of kidneys are not fundamentally and completely distinct, as earlier students (such as Semper) wrongly supposed; they represent three different generations of one and the same excretory apparatus; they correspond to three phylogenetic stages, and succeed each other in the stem-history of the Vertebrates in such wise that each younger and more advanced generation develops farther behind in the body, and replaces the older and less advanced generation that preceded it in time and space. The fore kidneys, first accurately described by Wilhelm Müller in 1875 in the Cyclostomes and Ichthyoda, form the sole excretory organ of the Acrania (Amphioxus); they continue in the Cyclostomes and some of the fishes, but are found only in slight traces and for a time in the embryos of the six other classes of Vertebrates. The primitive kidneys are first found in the Cyclostomes, behind the fore kidneys; they have been transmitted from the Selachii to all the Gnathostomes. In the Anamnia they act permanently as urinary glands; in the Amniotes their anterior part (“germinal kidneys”) changes into organs of the sexual apparatus, while the third generation develops from the end of their posterior part (“urinal kidneys”)—the characteristic after or permanent kidneys of the three higher classes of Vertebrates. The order in which the three renal systems succeed each other in the embryo of man and the higher Vertebrates corresponds to their phylogenetic succession in the history of our stem, and, consequently, in the natural classification of the Vertebrates.

Fig. 388—Pig-embryo, three-fifths of an inch long, seen from the ventral side. a fore leg, z hind leg, b ventral wall, r sexual prominence, w nephroduct, n primitive kidneys, n1 their inner part. (From Oscar Schultze.)

Fig. 389—Human embryo of the fifth week, two-fifths of an inch long, seen from the ventral side (the anterior ventral wall, b, is removed, the body-cavity, c, opened). d gut (cut off), f frontal process, g cerebrum, m middle brain, e after brain, h heart, k first gill-cleft, l pulmonary sac, n primitive kidneys, r sexual region, p phallus (sexual prominences), s tail. (From Kollmann.)

As in the morphology of any other system of organs, so in the case of the urinary and sexual organs the Amphioxus is the real typical primitive Vertebrate; it affords the key to the mysteries of the structure of man and the higher Vertebrates. The kidneys of the Amphioxus—first discovered by Boveri in 1890—are typical “fore kidneys,” composed of a double row of short segmental canals (Fig. 217 x). The inner aperture of these pronephridia opens into the mesodermic body-cavity (the middle part of the cœloma, B); the external aperture into the ectodermic mantle or peribranchial cavity (C). Their position, their structure, and their relation to the branchial vessel make it clear that these segmental pronephridia correspond to the rudimentary fore kidneys of the Craniotes. The mantle-cavity into which they open seems to correspond to the prorenal duct of the latter.

Figs. 390, 391, 392—Primitive kidneys and rudimentary sexual organs. Figs. 390 and 391 of Amphibia (frog-larvæ); Fig. 390 earlier, 391 later stage. Fig. 392 of a mammal (ox-embryo). u primitive kidney, k sexual gland (rudiment of testicle and ovary). The primary nephroduct (ug in Fig. 390) divides (in Figs. 391 and 392) into the two secondary nephroducts—the Mullerian (m) and Wolffian (ug′) ducts, joined together behind in the genital cord (g). l ligament of the primitive kidneys. (From Gegenbaur.)

Figs. 393, 394—Urinary and sexual organs of an Amphibian (water salamander or Triton). Fig. 393 of a female, 394 of a male. r primitive kidney, ov ovary, od oviduct and c Rathke’s duct, both developed from the Müllerian duct, u primitive ureter (also acting as spermaduct [ve] in the male, opening below into the Wolffian duct [u apostrophe]), ms mesovarium. (From Gegenbaur.)

The next higher Vertebrates, the Cyclostomes, yield some very interesting data. Both orders of this class, the hags and lampreys, have still the fore kidneys inherited from the Acrania—the former permanently, the latter in their earlier stages. Behind these the primitive kidneys soon develop, and in a very characteristic form. The remarkable structure of the mesonephros of the Cyclostomes, discovered by Johannes Müller, explains the intricate formation of the kidneys in the higher Vertebrates. We find in the hag-fishes (Bdellostoma) a long tube, the prorenal duct (nephroductus, Fig. 384 a). This opens with its anterior end into the cœloma by a ciliated aperture, and externally with its posterior end by an outlet in the skin. Inside it open a large number of small transverse canals (“segmental or primitive urinary canals,” b). Each of these terminates blindly in a vesicular capsule (c), and this encloses a coil of blood-vessel (glomerulus, an arterial network, Fig. 384 B, c). Afferent branches of arteries conduct arterial blood into the coiled branches of the glomerulus (d), and efferent arterial branches conduct it away from the net (c). The primitive renal canals (mesonephridia) are distinguished by this net-formation from their predecessors.

In the Selachii also we find a longitudinal row of segmental canals on each side, which open outwards into the primitive renal ducts (nephrotomes, p. 149. The segmental canals (a pair in each segment of the middle part of the body) open internally by a ciliated funnel into the body-cavity. From the posterior group of these organs a compact primitive kidney is formed, the anterior group taking part in the construction of the sexual organs.

In the same simple form that remains throughout life in the Myxinoides and partly in the Selachii we find the primitive kidney first developing in the embryo of man and the higher Craniotes (Figs. 386, 387). Of the two parts that compose the comb-shaped primitive kidney the longitudinal channel, or nephroduct, is always the first to appear; afterwards the transverse “canals,” the excreting nephridia, are formed in the mesoderm; and after this again the Malpighian capsules with their arterial coils are associated with these as cœlous outgrowths. The primitive renal duct, which appears first, is found in all craniote embryos at the early stage in which the differentiation of the medullary tube takes place in the ectoderm, the severance of the chorda from the visceral layer in the entoderm, and the first trace of the cœlom-pouches arises between the limiting layers (Fig. 385). The nephroduct (ung) is seen on each side, directly under the horny plate, in the shape of a long, thin, thread-like string of cells. It presently hollows out and becomes a canal, running straight from front to back, and clearly showing in the transverse section of the embryo its original position in the space between horny plate (h), primitive segments (uw), and lateral plates (hpl). As the originally very short urinary canals lengthen and multiply, each of the two primitive kidneys assumes the form of a half-feathered leaf (Fig. 387). The lines of the leaf are represented by the urinary canals (u), and the rib by the outlying nephroduct (w). At the inner edge of the primitive kidneys the rudiment of the ventral sexual gland (g) can now be seen as a body of some size. The hindermost end of the nephroduct opens right behind into the last section of the rectum, thus making a cloaca of it. However, this opening of the nephroducts into the intestine must be regarded as a secondary formation. Originally they open, as the Cyclostomes clearly show, quite independently of the gut, in the external skin of the abdomen.

Fig. 395—Primitive kidneys and germinal glands of a human embryo, three inches in length (beginning of the sixth week), magnified. k germinal gland, u primitive kidney, z diaphragmatic ligament of same, w Wolffian duct (opened on the right), g directing ligament (gubernaculum), a allantoic duct. (From Kollmann.)

In the Myxinoides the primitive kidneys retain this simple comb-shaped structure, and a part of it is preserved in the Selachii; but in all the other Craniotes it is only found for a short time in the embryo, as an ontogenetic reproduction of the earlier phylogenetic structure. In these the primitive kidney soon assumes the form (by the rapid growth, lengthening, increase, and serpentining of the urinary canals) of a large compact gland, of a long, oval or spindle-shaped character, which passes through the greater part of the embryonic body-cavity (Figs. 183 m, 184 m, 388 n). It lies near the middle line, directly under the primitive vertebral column, and reaches from the cardiac region to the cloaca. The right and left kidneys are parallel to each other, quite close together, and only separated by the mesentery—the thin narrow layer that attaches the middle gut to the under surface of the vertebral column. The passage of each primitive kidney, the nephroduct, runs towards the back on the lower and outer side of the gland, and opens in the cloaca, close to the starting-point of the allantois; it afterwards opens into the allantois itself.

The primitive or primordial kidneys of the amniote embryo were formerly called the “Wolffian bodies,” and sometimes “Oken’s bodies.” They act for a time as kidneys, absorbing unusable juices from the embryonic body and conducting them to the cloaca—afterwards to the allantois. There the primitive urine accumulates, and thus the allantois acts as bladder or urinary sac in the embryos of man and the other Amniotes. It has, however, no genetic connection with the primitive kidneys, but is a pouch-like growth from the anterior wall of the rectum (Fig. 147 u). Thus it is a product of the visceral layer, whereas the primitive kidneys are a product of the middle layer. Phylogenetically we must suppose that the allantois originated as a pouch-like growth from the cloaca-wall in consequence of the expansion caused by the urine accumulated in it and excreted by the kidneys. It is originally a blind sac of the rectum. The real bladder of the vertebrate certainly made its first appearance among the Dipneusts (in Lepidosiren), and has been transmitted from them to the Amphibia, and from these to the Amniotes. In the embryo of the latter it protrudes far out of the not yet closed ventral wall. It is true that many of the fishes also have a “bladder.” But this is merely a local enlargement of the lower section of the nephroducts, and so totally different in origin and composition from the real bladder. The two structures can be compared from the physiological point of view, and so are analogous, as they have the same function; but not from the morphological point of view, and are therefore not homologous. The false bladder of the fishes is a mesodermic product of the nephroducts; the true bladder of the Dipneusts, Amphibia, and Amniotes is an entodermic blind sac of the rectum.

Figs. 396–398—Urinary and sexual organs of ox-embryos. Fig. 396, female embryo one and a half inches long; Fig. 397, male embryo, one and a half inches long. Fig. 398 female embryo two and a half inches long. w primitive kidney, wg Wolffian duct, m Müllerian duct, m′ upper end of same (opened at t), i lower and thicker part of same (rudiment of uterus), g genital cord, h testicle, (h′, lower and h″, upper testicular ligament), o ovary, o′ lower ovarian ligament, i inguinal ligament of primitive kidney, d diaphragmatic ligament of primitive kidney, nn accessory kidneys, n permanent kidneys, under them the S-shaped ureters, between these the rectum, v bladder, a umbilical artery. (From Kölliker.)

In all the Anamnia (the lower amnionless Craniotes, Cyclostomes, Fishes, Dipneusts, and Amphibia) the urinary organs remain at a lower stage of development to this extent, that the primitive kidneys (protonephri) act permanently as urinary glands. This is only so as a passing phase of the early embryonic life in the three higher classes of Vertebrates, the Amniotes. In these the permanent or after or secondary (really tertiary) kidneys (renes or metanephri) that are distinctive of these three classes soon make their appearance. They represent the third and last generation of the vertebrate kidneys. The permanent kidneys do not arise (as was long supposed) as independent glands from the alimentary tube, but from the last section of the primitive kidneys and the nephroduct. Here a simple tube, the secondary renal duct, develops, near the point of its entry into the cloaca; and this tube grows considerably forward. With its blind upper or anterior end is connected a glandular renal growth, that owes its origin to a differentiation of the last part of the primitive kidneys. This rudiment of the permanent kidneys consists of coiled urinary canals with Malpighian capsules and vascular coils (without ciliated funnels), of the same structure as the segmental mesonephridia of the primitive kidneys. The further growth of these metanephridia gives rise to the compact permanent kidneys, which have the familiar bean-shape in man and most of the higher mammals, but consist of a number of separate folds in the lower mammals, birds, and reptiles. As the permanent kidneys grow rapidly and advance forward, their passage, the ureter, detaches altogether from its birth-place, the posterior end of the nephroduct; it passes to the posterior surface of the allantois. At first in the oldest Amniotes this ureter opens into the cloaca together with the last section of the nephroduct, but afterwards separately from this, and finally into the permanent bladder apart from the rectum altogether. The bladder originates from the hindmost and lowest part of the allantoic pedicle (urachus), which enlarges in spindle shape before the entry into the cloaca. The anterior or upper part of the pedicle, which runs to the navel in the ventral wall of the embryo, atrophies subsequently, and only a useless string-like relic of it is left as a rudimentary organ; that is the single vesico-umbilical ligament. To the right and left of it in the adult male are a couple of other rudimentary organs, the lateral vesico-umbilical ligaments. These are the degenerate string-like relics of the earlier umbilical arteries.

Though in man and all the other Amniotes the primitive kidneys are thus early replaced by the permanent kidneys, and these alone then act as urinary organs, all the parts of the former are by no means lost. The nephroducts become very important physiologically by being converted into the passages of the sexual glands. In all the Gnathostomes—or all the Vertebrates from the fishes up to man—a second similar canal develops beside the nephroduct at an early stage of embryonic evolution. The latter is usually called the Müllerian duct, after its discoverer, Johannes Müller, while the former is called the Wolffian duct. The origin of the Müllerian duct is still obscure; comparative anatomy and ontogeny seem to indicate that it originates by differentiation from the Wolffian duct. Perhaps it would be best to say: “The original primary nephroduct divides by differentiation (or longitudinal cleavage) into two secondary nephroducts, the Wolffian and the Müllerian ducts.” The latter (Fig. 387 m) lies just on the inner side of the former (Fig. 387 w). Both open behind into the cloaca.

Fig. 399—Female sexual organs of a Monotreme (Ornithorhynchus, Fig. 269). o ovaries, t oviducts, u womb, sug urogenital sinus; at u′ is the outlet of the two wombs, and between them the bladder (vu). cl cloaca. (From Gegenbaur.)

However uncertain the origin of the nephroduct and its two products, the Müllerian and the Wolffian ducts, may be, its later development is clear enough. In all the Gnathostomes the Wolffian duct is converted into the spermaduct, and the Müllerian duct into the oviduct. Only one of them is retained in each sex; the other either disappears altogether, or only leaves relics in the shape of rudimentary organs. In the male sex, in which the two Wolffian ducts become the spermaducts, we often find traces of the Müllerian ducts, which I have called “Rathke’s canals” (Fig. 394 c). In the female sex, in which the two Müllerian ducts form the oviducts, there are relics of the Wolffian ducts, which are called “the ducts of Gaertner.”

We obtain the most interesting information with regard to this remarkable evolution of the nephroducts and their association with the sexual glands from the Amphibia (Figs. 390–395). The first structure of the nephroduct and its differentiation into Müllerian and Wolffian ducts are just the same in both sexes in the Amphibia, as in the mammal embryos (Figs. 392, 396). In the female Amphibia the Müllerian duct develops on either side into a large oviduct (Fig. 393 od), while the Wolffian duct acts permanently as ureter (u). In the male Amphibia the Müllerian duct only remains as a rudimentary organ without any functional significance, as Rathke’s canal (Fig. 394 c); the Wolffian duct serves also as ureter, but at the same time as spermaduct, the sperm-canals (ve) that proceed from the testicles (t) entering the fore part of the primitive kidneys and combining there with the urinary canals.

Figs. 400, 401—Original position of the sexual glands in the ventral cavity of the human embryo (three months old). Fig. 400, male. h testicles, gh conducting ligament of the testicles, wg spermaduct, h bladder, uh inferior vena cava, nn accessory kidneys, n kidneys. Fig. 401, female. r round maternal ligament (underneath it the bladder, over it the ovaries). r′ kidneys, s accessory kidneys, c cæcum, o small reticle, om large reticle (stomach between the two), l spleen. (From Kölliker.)

In the mammals these permanent amphibian features are only seen as brief phases of the earlier period of embryonic development (Fig. 392). Here the primitive kidneys, which act as excretory organs of urine throughout life in the amnion-less Vertebrates, are replaced in the mammals by the permanent kidneys. The real primitive kidneys disappear for the most part at an early stage of development, and only small relics of them remain. In the male mammal the epididymis develops from the uppermost part of the primitive kidney; in the female a useless rudimentary organ, the epovarium, is formed from the same part. The atrophied relic of the former is known as the paradidymis, that of the latter as the parovarium.

Fig. 402—Urogenital system of a human embryo of three inches in length. h testicles, wg spermaducts, gh conducting ligament, p processus vaginalis, b bladder, au umbilical arteries, m mesorchium, d intestine, u ureter, n kidney, nn accessory kidney. (From Kollman.)

The Müllerian ducts undergo very important changes in the female mammal. The oviducts proper are developed only from their upper part; the lower part dilates into a spindle-shaped tube with thick muscular wall, in which the impregnated ovum develops into the embryo. This is the womb (uterus). At first the two wombs (Fig. 399 u) are completely separate, and open into the cloaca on either side of the bladder (vu), as is still the case in the lowest living mammals, the Monotremes. But in the Marsupials a communication is opened between the two Müllerian ducts, and in the Placentals they combine below with the rudimentary Wolffian ducts to form a single “genital cord.” The original independence of the two wombs and the vaginal canals formed from their lower ends are retained in many of the lower Placentals, but in the higher they gradually blend and form a single organ. The conjunction proceeds from below (or behind) upwards (or forwards). In many of the Rodents (such as the rabbit and squirrel) two separate wombs still open into the simple and single vaginal canal; but in others, and in the Carnivora, Cetacea, and Ungulates, the lower halves of the wombs have already fused into a single piece, though the upper halves (or “horns”) are still separate (“two-horned” womb, uteris bicornis). In the bats and lemurs the “horns” are very short, and the lower common part is longer. Finally, in the apes and in man the blending of the two halves is complete, and there is only the one simple, pear-shaped uterine pouch, into which the oviducts open on each side. This simple uterus is a late evolutionary product, and is found only in the ape and man.

Figs. 403–406—Origin of human ova in the female ovary. Fig. 403. Vertical section of the ovary of a new-born female infant, a ovarian epithelium, b rudimentary string of ova, c young ova in the epithelium, d long string of ova with follicle-formation (Pflüger’s tube), e group of young follicles, f isolated young follicle, g blood-vessels in connective tissue (stroma) of the ovary. In the strings the young ova are distinguished by their considerable size from the surrounding follicle-cells. (From Waldeyer.)
Fig. 404—Two young Graafian follicles, isolated. In 1 the follicle-cells still form a simple, and in 2 a double, stratum round the young ovum; in 2 they are beginning to form the ovolemma or the zona pellucida (a).
Figs. 405 and 406—Two older Graafian follicles, in which fluid is beginning to accumulate inside the eccentrically thickened epithelial mass of the follicle-cells (Fig. 405 with little, 406 with much, follicle-water). ei the young ovum, with embryonic vesicle and spot, zp ovolemma or zona pellucida, dp discus proligerus, formed of an accumulation of follicle-cells, which surround the ovum, ff follicle-liquid (liquor folliculi), gathered inside the stratified follicle-epithelium (fe), fk connective-tissue fibrous capsule of the Graafian follicle (theca folliculi).

In the male mammals there is the same fusion of the Müllerian and Wolffian ducts at their lower ends. Here again they form a single genital cord (Fig. 397 g), and this opens similarly into the original urogenital sinus, which develops from the lowest section of the bladder (v). But while in the male mammal the Wolffian ducts develop into the permanent spermaducts, there are only rudimentary relics left of the Müllerian ducts. The most notable of these is the “male womb” (uterus masculinus), which originates from the lowest fused part of the ducts, and corresponds to the female uterus. It is a small, flask-shaped vesicle without any physiological significance, which opens into the ureter between the two spermaducts and the prostate folds (vesicula prostatica).

Fig. 407—A ripe human Graafian follicle. a the mature ovum, b the surrounding follicle-cells, c the epithelial cells of the follicle, d the fibrous membrane of the follicle, e its outer surface.

The internal sexual organs of the mammals undergo very distinctive changes of position. At first the germinal glands of both sexes lie deep inside the ventral cavity, at the inner edge of the primitive kidneys (Figs. 386 g, 392 k), attached to the vertebral column by a short mesentery (mesorchium in the male, mesovarium in the female). But this primary arrangement is retained permanently only in the Monotremes (and the lower Vertebrates). In all other mammals (both Marsupials and Placentals) they leave their original cradle and travel more or less far down (or behind), following the direction of a ligament that goes from the primitive kidneys to the inguinal region of the ventral wall. This is the inguinal ligament of the primitive kidneys, known in the male as the Hunterian ligament (Fig. 400 gh), and in the female as the “round maternal ligament” (Fig. 401 r). In woman the ovaries travel more or less towards the small pelvis, or enter into it altogether. In the male the testicles pass out of the ventral cavity, and penetrate by the inguinal canal into a sac-shaped fold of the outer skin. When the right and left folds (“sexual swellings”) join together they form the scrotum. The various mammals bring before us the successive stages of this displacement. In the elephant and the whale the testicles descend very little, and remain underneath the kidneys. In many of the rodents and carnassia they enter the inguinal canal. In most of the higher mammals they pass through this into the scrotum. As a rule, the inguinal canal closes up. When it remains open the testicles may periodically pass into the scrotum, and withdraw into the ventral cavity again in time of rut (as in many of the marsupials, rodents, bats, etc.).

The structure of the external sexual organs, the copulative organs that convey the fecundating sperm from the male to the female organism in the act of copulation, is also peculiar to the mammals. There are no organs of this character in most of the other Vertebrates. In those that live in water (such as the Acrania and Cyclostomes, and most of the fishes) the ova and sperm-cells are simply ejected into the water, where their conjunction and fertilisation are left to chance. But in many of the fishes and amphibia, which are viviparous, there is a direct conveyance of the male sperm into the female body; and this is the case with all the Amniotes (reptiles, birds, and mammals). In these the urinary and sexual organs always open originally into the last section of the rectum, which thus forms a cloaca (p. 249). Among the mammals this arrangement is permanent only in the Monotremes, which take their name from it (Fig. 399 cl). In all the other mammals a frontal partition is developed in the cloaca (in the human embryo about the beginning of the third month), and this divides it into two cavities. The anterior cavity receives the urogenital canal, and is the sole outlet of the urine and the sexual products; the hind or anus-cavity passes the excrements only.

Even before this partition has been formed in the Marsupials and Placentals, we see the first trace of the external sexual organs. First a conical protuberance rises at the anterior border of the cloaca-outlet—the sexual prominence (phallus, Fig. 402 A, e, B, e). At the tip it is swollen in the shape of a club (“acorn” glans). On its under side there is a furrow, the sexual groove (sulcus genitalis, f), and on each side of this a fold of skin, the “sexual pad” (torus genitalis, h l). The sexual protuberance or phallus is the chief organ of the sexual sense (p. 282); the sexual nerves spread on it, and these are the principal organs of the specific sexual sensation. As erectile bodies (corpora cavernosa) are developed in the male phallus by peculiar modifications of the blood-vessels, it becomes capable of erecting periodically on a strong accession of blood, becoming stiff, so as to penetrate into the female vagina and thus effect copulation. In the male the phallus becomes the penis; in the female it becomes the much smaller clitoris; this is only found to be very large in certain apes (Ateles). A prepuce (“foreskin”) is developed in both sexes as a protecting fold on the anterior surface of the phallus.

Fig. 408—The human ovum after issuing from the Graafian follicle, surrounded by the clinging cells of the discus proligerus (in two radiating crowns). z ovolemma (zona pellucida, with radial porous canals), p cytosoma (protoplasm of the cell-body, darker within, lighter without), k nucleus of the ovum (embryonic vesicle). (From Nagel.) (Cf. Figs. 1 and 14.)

The external sexual member (phallus) is found at various stages of development within the mammal class, both in regard to size and shape, and the differentiation and structure of its various parts; this applies especially to the terminal part of the phallus, the glans, both the larger glans penis of the male and the smaller glans clitoridis of the female. The part of the cloaca from the upper wall of which it forms belongs to the proctodæum, the ectodermic invagination of the rectum (p. 311); hence its epithelial covering can develop the same horny growths as the corneous layer of the epidermis. Thus the glans, which is quite smooth in man and the higher apes, is covered with spines in many of the lower apes and in the cat, and in many of the rodents with hairs (marmot) or scales (guinea-pig) or solid horny warts (beaver). Many of the Ungulates have a free conical projection on the glans, and in many of the Ruminants this “phallus-tentacle” grows into a long cone, bent hook-wise at the base (as in the goat, antelope, gazelle, etc.). The different forms of the phallus are connected with variations in the structure and distribution of the sensory corpuscles—i.e. the real organs of the sexual sense, which develop in certain papillæ of the corium of the phallus, and have been evolved from ordinary tactile corpuscles of the corium by erotic adaptation (p. 282).

The formation of the corpora cavernosa, which cause the stiffness of the phallus and its capability of penetrating the vagina, by certain special structures of their spongy vascular spaces, also shows a good deal of variety within the vertebrate stem. This stiffness is increased in many orders of mammals (especially the carnassia and rodents) by the ossification of a part of the fibrous body (corpus fibrosum). This penis-bone (os priapi) is very large in the badger and dog, and bent like a hook in the marten; it is also very large in some of the lower apes, and protrudes far out into the glans. It is wanting in most of the anthropoid apes; it seems to have been lost in their case (and in man) by atrophy.

The sexual groove on the under side of the phallus receives in the male the mouth of the urogenital canal, and is changed into a continuation of this, becoming a closed canal by the juncture of its parallel edges, the male urethra. In the female this only takes place in a few cases (some of the lemurs, rodents, and moles); as a rule, the groove remains open, and the borders of this “vestibule of the vagina” develop into the smaller labia (nymphæ). The large labia of the female develop from the sexual pads (tori genitales), the two parallel folds of the skin that are found on each side of the genital groove. They join together in the male, and form the closed scrotum. These striking differences between the two sexes cannot yet be detected in the human embryo of the ninth week. We begin to trace them in the tenth week of development, and they are accentuated in proportion as the difference of the sexes develops.

Sometimes the normal juncture of the two sexual pads in the male fails to take place, and the sexual groove may also remain open (hypospadia). In these cases the external male genitals resemble the female, and they are often wrongly regarded as cases of hermaphrodism. Other malformations of various kinds are not infrequently found in the human external sexual organs, and some of them have a great morphological interest. The reverse of hypospadia, in which the penis is split open below, is seen in epispadia, in which the urethra is open above. In this case the urogenital canal opens above at the dorsal root of the penis; in the former case down below. These and similar obstructions interfere with a man’s generative power, and thus prejudicially affect his whole development. They clearly prove that our history is not guided by a “kind Providence,” but left to the play of blind chance.

We must carefully distinguish the rarer cases of real hermaphrodism from the preceding. This is only found when the essential organs of reproduction, the genital glands of both kinds, are united in one individual. In these cases either an ovary is developed on the right and a testicle on the left (or vice versa); or else there are testicles and ovaries on both sides, some more and others less developed. As hermaphrodism was probably the original arrangement in all the Vertebrates, and the division of the sexes only followed by later differentiation of this, these curious cases offer no theoretical difficulty. But they are rarely found in man and the higher mammals. On the other hand, we constantly find the original hermaphrodism in some of the lower Vertebrates, such as the Myxinoides, many fishes of the perch-type (serranus), and some of the Amphibia (ringed snake, toad). In these cases the male often has a rudimentary ovary at the fore end of the testicle; and the female sometimes has a rudimentary, inactive testicle. In the carp also and some other fishes this is found occasionally. We have already seen how traces of the earlier hemaphrodism can be traced in the passages of the Amphibia.

Man has faithfully preserved the main features of his stem-history in the ontogeny of his urinary and sexual organs. We can follow their development step by step in the human embryo in the same advancing gradation that is presented to us by the comparison of the urogenital organs in the Acrania, Cyclostomes; Fishes, Amphibia, Reptiles, and then (within the mammal series) in the Monotremes, Marsupials, and the various Placentals. All the peculiarities of urogenital structure that distinguish the mammals from the rest of the Vertebrates are found in man; and in all special structural features he resembles the apes, particularly the anthropoid apes. In proof of the fact that the special features of the mammals have been inherited by man, I will, in conclusion, point out the identical way in which the ova are formed in the ovary. In all the mammals the mature ova are contained in special capsules, which are known as the Graafian follicles, after their discoverer, Roger de Graaf (1677). They were formerly supposed to be the ova themselves; but Baer discovered the ova within the follicles (p. 16). Each follicle (Fig. 407) consists of a round fibrous capsule (d), which contains fluid and is lined with several strata of cells (c). The layer is thickened like a knob at one point (b); this ovum-capsule encloses the ovum proper (a). The mammal ovary is originally a very simple oval body (Fig. 387 g), formed only of connective tissue and blood-vessels, covered with a layer of cells, the ovarian epithelium or the female germ epithelium. From this germ epithelium strings of cells grow out into the connective tissue or “stroma” of the ovary (Fig. 403 b). Some of the cells of these strings (or Pflüger’s tubes) grow larger and become ova (primitive ova, c); but the great majority remain small, and form a protective and nutritive stratum of cells round each ovum—the “follicle-epithelium” (e).

The follicle-epithelium of the mammal has at first one stratum (Fig. 404 1), but afterwards several (2). It is true that in all the other Vertebrates the ova are enclosed in a membrane, or “follicle,” that consists of smaller cells. But it is only in the mammals that fluid accumulates between the growing follicle-cells, and distends the follicle into a large round capsule, on the inside wall of which the ovum lies, at one side (Figs. 405, 406). There again, as in the whole of his morphology, man proves indubitably his descent from the mammals.

In the lower Vertebrates the formation of ova in the germ-epithelium of the ovary continues throughout life; but in the higher it is restricted to the earlier stages, or even to the period of embryonic development. In man it seems to cease in the first year; in the second year we find no new-formed ova or chains of ova (Pflüger’s tubes). However, the number of ova in the two ovaries is very large in the young girl; there are calculated to be 72,000 in the sexually-mature maiden. In the production of the ova men resemble most of the anthropoid apes.

Generally speaking, the natural history of the human sexual organs is one of those parts of anthropology that furnish the most convincing proofs of the animal origin of the human race. Any man who is acquainted with the facts and impartially weighs them will conclude from them alone that we have been evolved from the lower Vertebrates. The larger and the detailed structure, the action, and the embryological development of the sexual organs are just the same in man as in the apes. This applies equally to the male and the female, the internal and the external organs. The differences we find in this respect between man and the anthropoid apes are much slighter than the differences between the various species of apes. But all the apes have certainly a common origin, and have been evolved from a long-extinct early-Tertiary stem-form, which we must trace to a branch of the lemurs. If we had this unknown pithecoid stem-form before us, we should certainly put it in the order of the true apes in the primate system; but within this order we cannot, for the anatomic and ontogenetic reasons we have seen, separate man from the group of the anthropoid apes. Here again, therefore, on the ground of the pithecometra-principle, comparative anatomy and ontogeny teach with full confidence the descent of man from the ape.

Chapter XXX.
RESULTS OF ANTHROPOGENY

Now that we have traversed the wonderful region of human embryology and are familiar with the principal parts of it, it will be well to look back on the way we have come, and forward to the further path to truth to which it has led us. We started from the simplest facts of ontogeny, or the development of the individual—from observations that we can repeat and verify by microscopic and anatomic study at any moment. The first and most important of these facts is that every man, like every other animal, begins his existence as a simple cell. This round ovum has the same characteristic form and origin as the ovum of any other mammal. From it is developed in the same manner in all the Placentals, by repeated cleavage, a multicellular blastula. This is converted into a gastrula, and this in turn into a blastocystis (or embryonic vesicle). The two strata of cells that compose its wall are the primary germinal layers, the skin-layer (ectoderm), and gut-layer (entoderm). This two-layered embryonic form is the ontogenetic reproduction of the extremely important phylogenetic stem-form of all the Metazoa, which we have called the Gastræa. As the human embryo passes through the gastrula-form like that of all the other Metazoa, we can trace its phylogenetic origin to the Gastræa.

As we continued to follow the embryonic development of the two-layered structure, we saw that first a third, or middle layer (mesoderm), appears between the two primary layers; when this divides into two, we have the four secondary germinal layers. These have just the same composition and genetic significance in man as in all the other Vertebrates. From the skin-sense layer are developed the epidermis, the central nervous system, and the chief part of the sense-organs. The skin-fibre layer forms the corium and the motor organs—the skeleton and the muscular system. From the gut-fibre layer are developed the vascular system, the muscular wall of the gut, and the sexual glands. Finally, the gut-gland layer only forms the epithelium, or the inner cellular stratum of the mucous membrane of the alimentary canal and glands (lungs, liver, etc.).

The manner in which these different systems of organs arise from the secondary germinal layers is essentially the same from the start in man as in all the other Vertebrates. We saw, in studying the embryonic development of each organ, that the human embryo follows the special lines of differentiation and construction that are only found otherwise in the Vertebrates. Within the limits of this vast stem we have followed, step by step, the development both of the body as a whole and of its various parts. This higher development follows in the human embryo the form that is peculiar to the mammals. Finally, we saw that, even within the limits of this class, the various phylogenetic stages that we distinguish in a natural classification of the mammals correspond to the ontogenetic stages that the human embryo passes through in the course of its evolution. We were thus in a position to determine precisely the position of man in this class, and so to establish his relationship to the different orders of mammals.

The line of argument we followed in this explanation of the ontogenetic facts was simply a consistent application of the biogenetic law. In this we have throughout taken strict account of the distinction between palingenetic and cenogenetic phenomena. Palingenesis (or “synoptic development”) alone enables us to draw conclusions from the observed embryonic form to the stem-form preserved by heredity. Such inference becomes more or less precarious when there has been cenogenesis, or disturbance of development, owing to fresh adaptations. We cannot understand embryonic development unless we appreciate this very important distinction. Here we stand at the very limit that separates the older and the new science or philosophy of nature. The whole of the results of recent morphological research compel us irresistibly to recognise the biogenetic law and its far-reaching consequences. These are, it is true, irreconcilable with the legends and doctrines of former days, that have been impressed on us by religious education. But without the biogenetic law, without the distinction between palingenesis and cenogenesis, and without the theory of evolution on which we base it, it is quite impossible to understand the facts of organic development; without them we cannot cast the faintest gleam of explanation over this marvellous field of phenomena. But when we recognise the causal correlation of ontogeny and phylogeny expressed in this law, the wonderful facts of embryology are susceptible of a very simple explanation; they are found to be the necessary mechanical effects of the evolution of the stem, determined by the laws of heredity and adaptation. The correlative action of these laws under the universal influence of the struggle for existence, or—as we may say in a word, with Darwin—“natural selection,” is entirely adequate to explain the whole process of embryology in the light of phylogeny. It is the chief merit of Darwin that he explained by his theory of selection the correlation of the laws of heredity and adaptation that Lamarck had recognised, and pointed out the true way to reach a causal interpretation of evolution.

The phenomenon that it is most imperative to recognise in this connection is the inheritance of functional variations. Jean Lamarck was the first to appreciate its fundamental importance in 1809, and we may therefore justly give the name of Lamarckism to the theory of descent he based on it. Hence the radical opponents of the latter have very properly directed their attacks chiefly against the former. One of the most distinguished and most narrow-minded of these opponents, Wilhelm His, affirms very positively that “characteristics acquired in the life of the individual are not inherited.”

The inheritance of acquired characters is denied, not only by thorough opponents of evolution, but even by scientists who admit it and have contributed a good deal to its establishment, especially Weismann, Galton, Ray Lankester, etc. Since 1884 the chief opponent has been August Weismann, who has rendered the greatest service in the development of Darwin’s theory of selection. In his work on The Continuity of the Germ-plasm, and in his recent excellent Lectures on the Theory of Descent (1902), he has with great success advanced the opinion that “only those characters can be transmitted to subsequent generations that were contained in rudimentary form in the embryo.” However, this germ-plasm theory, with its attempt to explain heredity, is merely a “provisional molecular hypothesis”; it is one of those metaphysical speculations that attribute the evolutionary phenomena exclusively to internal causes, and regard the influence of the environment as insignificant. Herbert Spencer, Theodor Eimer, Lester Ward, Hering, and Zehnder have pointed out the untenable consequences of this position. I have given my view of it in the tenth edition of the History of Creation (pp. 192, 203). I hold, with Lamarck and Darwin, that the hereditary transmission of acquired characters is one of the most important phenomena in biology, and is proved by thousands of morphological and physiological experiences. It is an indispensable foundation of the theory of evolution.

Of the many and weighty arguments for the truth of this conception of evolution I will for the moment merely point to the invaluable evidence of dysteleology, the science of rudimentary organs. We cannot insist too often or too strongly on the great morphological significance of these remarkable organs, which are completely useless from the physiological point of view. We find some of these useless parts, inherited from our lower vertebrate ancestors, in every system of organs in man and the higher Vertebrates. Thus we find at once on the skin a scanty and rudimentary coat of hair, only fully developed on the head, under the shoulders, and at a few other parts of the body. The short hairs on the greater part of the body are quite useless and devoid of physiological value; they are the last relic of the thicker hairy coat of our simian ancestors. The sensory apparatus presents a series of most remarkable rudimentary organs. We have seen that the whole of the shell of the external ear, with its cartilages, muscles, and skin, is in man a useless appendage, and has not the physiological importance that was formerly ascribed to it. It is the degenerate remainder of the pointed, freely moving, and more advanced mammal ear, the muscles of which we still have, but cannot work them. We found at the inner corner of our eye a small, curious, semi-lunar fold that is of no use whatever to us, and is only interesting as the last relic of the nictitating membrane, the third, inner eye-lid that had a distinct physiological purpose in the ancient sharks, and still has in many of the Amniotes.

The motor apparatus, in both the skeleton and muscular systems, provides a number of interesting dysteleological arguments. I need only recall the projecting tail of the human embryo, with its rudimentary caudal vertebræ and muscles; this is totally useless in man, but very interesting as the degenerate relic of the long tail of our simian ancestors. From these we have also inherited various bony processes and muscles, which were very useful to them in climbing trees, but are useless to us. At various points of the skin we have cutaneous muscles which we never use—remnants of a strongly-developed cutaneous muscle in our lower mammal ancestors. This “panniculus carnosus” had the function of contracting and creasing the skin to chase away the flies, as we see every day in the horse. Another relic in us of this large cutaneous muscle is the frontal muscle, by which we knit our forehead and raise our eye-brows; but there is another considerable relic of it, the large cutaneous muscle in the neck (platysma myoides), over which we have no voluntary control.

Not only in the systems of animal organs, but also in the vegetal apparatus, we find a number of rudimentary organs, many of which we have already noticed. In the alimentary apparatus there are the thymus-gland and the thyroid gland, the seat of goitre and the relic of a ciliated groove that the Tunicates and Acrania still have in the gill-pannier; there is also the vermiform appendix to the cæcum. In the vascular system we have a number of useless cords which represent relics of atrophied vessels that were once active as blood-canals—the ductus Botalli between the pulmonary artery and the aorta, the ductus venosus Arantii between the portal vein and the vena cava, and many others. The many rudimentary organs in the urinary and sexual apparatus are particularly interesting. These are generally developed in one sex and rudimentary in the other. Thus the spermaducts are formed from the Wolffian ducts in the male, whereas in the female we have merely rudimentary traces of them in Gaertner’s canals. On the other hand, in the female the oviducts and womb are developed from the Mullerian ducts, while in the male only the lowest ends of them remain as the “male womb” (vesicula prostatica). Again, the male has in his nipples and mammary glands the rudiments of organs that are usually active only in the female.

A careful anatomic study of the human frame would disclose to us numbers of other rudimentary organs, and these can only be explained on the theory of evolution. Robert Wiedersheim has collected a large number of them in his work on The Human Frame as a Witness to its Past. They are some of the weightiest proofs of the truth of the mechanical conception and the strongest disproofs of the teleological view. If, as the latter demands, man or any other organism had been designed and fitted for his life-purposes from the start and brought into being by a creative act, the existence of these rudimentary organs would be an insoluble enigma; it would be impossible to understand why the Creator had put this useless burden on his creatures to walk a path that is in itself by no means easy. But the theory of evolution gives the simplest possible explanation of them. It says: The rudimentary organs are parts of the body that have fallen into disuse in the course of centuries; they had definite functions in our animal ancestors, but have lost their physiological significance. On account of fresh adaptations they have become superfluous, but are transmitted from generation to generation by heredity, and gradually atrophy.

We have inherited not only these rudimentary parts, but all the organs of our body, from the mammals—proximately from the apes. The human body does not contain a single organ that has not been inherited from the apes. In fact, with the aid of our biogenetic law we can trace the origin of our various systems of organs much further, down to the lowest stages of our ancestry. We can say, for instance, that we have inherited the oldest organs of the body, the external skin and the internal coat of the alimentary system, from the Gastræads; the nervous and muscular systems from the Platodes; the vascular system, the body-cavity, and the blood from the Vermalia; the chorda and the branchial gut from the Prochordonia; the articulation of the body from the Acrania; the primitive skull and the higher sense-organs from the Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot from the Amphibia; the palate from the Reptiles; the hairy coat, the mammary glands, and the external sexual organs from the Pro-mammals. When we formulated “the law of the ontogenetic connection of systematically related forms,” and determined the relative age of organs, we saw how it was possible to draw phylogenetic conclusions from the ontogenetic succession of systems of organs.

With the aid of this important law and of comparative anatomy we were also enabled to determine “man’s place in nature,” or, as we put it, assign to man his position in the classification of the animal kingdom. In recent zoological classification the animal world is divided into twelve stems or phyla, and these are broadly sub-divided into about sixty classes, and these classes into at least 300 orders. In his whole organisation man is most certainly, in the first place, a member of one of these stems, the vertebrate stem; secondly, a member of one particular class in this stem, the Mammals; and thirdly, of one particular order, the order of Primates. He has all the characteristics that distinguish the Vertebrates from the other eleven animal stems, the Mammals from the other sixty classes, and the Primates from the 300 other orders of the animal kingdom. We may turn and twist as we like, but we cannot get over this fact of anatomy and classification. Of late years this fact has given rise to a good deal of discussion, and especially of controversy as to the particular anatomic relationship of man to the apes. The most curious opinions have been advanced on this “ape-question,” or “pithecoid-theory.” It is as well, therefore, to go into it once more and distinguish the essential from the unessential. (Cf. pp. 261–5.)

We start from the undisputed fact that man is in any case—whether we accept or reject his special blood-relationship to the apes—a true mammal; in fact, a placental mammal. This fundamental fact can be proved so easily at any moment from comparative anatomy that it has been universally admitted since the separation of the Placentals from the lower mammals (Marsupials and Monotremes). But for every consistent subscriber to the theory of evolution it must follow at once that man descends from a common stem-form with all the other Placentals, the stem-ancestor of the Placentals, just as we must admit a common mesozoic ancestor of all the mammals. This is, however, to settle decisively the great and burning question of man’s place in nature, whether or no we go on to admit a nearer or more distant relationship to the apes. Whether man is or is not a member of the ape-order (or, if you prefer, the primate-order.) in the phylogenetic sense, in any case his direct blood-relationship to the rest of the mammals, and especially the Placentals, is established. It is possible that the affinities of the various orders of mammals to each other are different from what we hypothetically assume to-day. But, in any case, the common descent of man and all the other mammals from one stem-form is beyond question. This long-extinct Promammal was probably evolved from Proreptiles during the Triassic period, and must certainly be regarded as the monotreme and oviparous ancestor of all the mammals.

If we hold firmly to this fundamental and most important thesis, we shall see the “ape-question” in a very different light from that in which it is usually regarded. Little reflection is then needed to see that it is not nearly so important as it is said to be. The origin of the human race from a series of mammal ancestors, and the historic evolution of these from an earlier series of lower vertebrate ancestors, together with all the weighty conclusions that every thoughtful man deduces therefrom, remain untouched; so far as these are concerned, it is immaterial whether we regard true “apes” as our nearest ancestors or not. But as it has become the fashion to lay the chief stress in the whole question of man’s origin on the “descent from the apes,” I am compelled to return to it once more, and recall the facts of comparative anatomy and ontogeny that give a decisive answer to this “ape-question.”

The shortest way to attain our purpose is that followed by Huxley in 1863 in his able work, which I have already often quoted, Man’s Place in Nature—the way of comparative anatomy and ontogeny. We have to compare impartially all man’s organs with the same organs in the higher apes, and then to examine if the differences between the two are greater than the corresponding differences between the higher and the lower apes. The indubitable and incontestable result of this comparative-anatomical study, conducted with the greatest care and impartiality, was the pithecometra-principle, which we have called the Huxleian law in honour of its formulator—namely, that the differences in organisation between man and the most advanced apes we know are much slighter than the corresponding differences in organisation between the higher and lower apes. We may even give a more precise formula to this law, by excluding the Platyrrhines or American apes as distant relatives, and restricting the comparison to the narrower family-circle of the Catarrhines, the apes of the Old World. Within the limits of this small group of mammals we found the structural differences between the lower and higher catarrhine apes—for instance, the baboon and the gorilla—to be much greater than the differences between the anthropoid apes and man. If we now turn to ontogeny, and find, according to our “law of the ontogenetic connection of systematically related forms,” that the embryos of the anthropoid apes and man retain their resemblance for a longer time than the embryos of the highest and the lowest apes, we are forced, whether we like it or no, to recognise our descent from the order of apes. We can assuredly construct an approximate picture in the imagination of the form of our early Tertiary ancestors from the foregoing facts of comparative anatomy; however we may frame this in detail, it will be the picture of a true ape, and a distinct catarrhine ape. This has been shown so well by Huxley (1863) that the recent attacks of Klaatsch, Virchow, and other anthropologists, have completely failed (cf. pp.263–264). All the structural characters that distinguish the Catarrhines from the Platyrrhines are found in man. Hence in the genealogy of the mammals we must derive man immediately from the catarrhine group, and locate the origin of the human race in the Old World. Only the early root-form from which both descended was common to them.

It is, therefore, established beyond question for all impartial scientific inquiry that the human race comes directly from the apes of the Old World; but, at the same time, I repeat that this is not so important in connection with the main question of the origin of man as is commonly supposed. Even if we entirely ignore it, all that we have learned from the zoological facts of comparative anatomy and ontogeny as to the placental character of man remains untouched. These prove beyond all doubt the common descent of man and all the rest of the mammals. Further, the main question is not in the least affected if it is said: “It is true that man is a mammal; but he has diverged at the very root of the class from all the other mammals, and has no closer relationship to any living group of mammals.” The affinity is more or less close in any case, if we examine the relation of the mammal class to the sixty other classes of the animal world. Quite certainly the whole of the mammals, including man, have had a common origin; and it is equally certain that their common stem-forms were gradually evolved from a long series of lower Vertebrates.

The resistance to the theory of a descent from the apes is clearly due in most men to feeling rather than to reason. They shrink from the notion of such an origin just because they see in the ape organism a caricature of man, a distorted and unattractive image of themselves, because it hurts man’s æsthetic complacency and self-ennoblement. It is more flattering to think we have descended from some lofty and god-like being; and so, from the earliest times, human vanity has been pleased to believe in our origin from gods or demi-gods. The Church, with that sophistic reversal of ideas of which it is a master, has succeeded in representing this ridiculous piece of vanity as “Christian humility”; and the very men who reject with horror the notion of an animal origin, and count themselves “children of God,” love to prate of their “humble sense of servitude.” In most of the sermons that have poured out from pulpit and altar against the doctrine of evolution human vanity and conceit have been a conspicuous element; and, although we have inherited this very characteristic weakness from the apes, we must admit that we have developed it to a higher degree, which is entirely repudiated by sound and normal intelligence. We are greatly amused at all the childish follies that the ridiculous pride of ancestry has maintained from the Middle Ages to our own time; yet there is a large amount of this empty feeling in most men. Just as most people much prefer to trace their family back to some degenerate baron or some famous prince rather than to an unknown peasant, so most men would rather have as parent of the race a sinful and fallen Adam than an advancing, and vigorous ape. It is a matter of taste, and to that extent we cannot quarrel over these genealogical tendencies. Personally, the notion of ascent is more congenial to me than that of descent. It seems to me a finer thing to be the advanced offspring of a simian ancestor, that has developed progressively from the lower mammals in the struggle for life, than the degenerate descendant of a god-like being, made from a clod, and fallen for his sins, and an Eve created from one of his ribs. Speaking of the rib, I may add to what I have said about the development of the skeleton, that the number of ribs is just the same in man and woman. In both of them the ribs are formed from the middle germinal layer, and are, from the phylogenetic point of view, lower or ventral vertebral arches.

But it is said: “That is all very well, as far as the human body is concerned; on the facts quoted it is impossible to doubt that it has really and gradually been evolved from the long ancestral series of the Vertebrates. But it is quite another thing as regards man’s mind, or soul; this cannot possibly have been developed from the vertebrate-soul.”[^[35]] Let us see if we cannot meet this grave stricture from the well-known facts of comparative anatomy, physiology, and embryology. It will be best to begin with a comparative study of the souls of various groups of Vertebrates. Here we find such an enormous variety of vertebrate souls that, at first sight, it seems quite impossible to trace them all to a common “Primitive Vertebrate.” Think of the tiny Amphioxus, with no real brain but a simple medullary tube, and its whole psychic life at the very lowest stage among the Vertebrates. The following group of the Cyclostomes are still very limited, though they have a brain. When we pass on to the fishes, we find their intelligence remaining at a very low level. We do not see any material advance in mental development until we go on to the Amphibia and Reptiles. There is still greater advance when we come to the Mammals, though even here the minds of the Monotremes and of the stupid Marsupials remain at a low stage. But when we rise from these to the Placentals we find within this one vast group such a number of important stages of differentiation and progress that the psychic differences between the least intelligent (such as the sloths and armadillos) and the most intelligent Placentals (such as the dogs and apes) are much greater than the psychic differences between the lowest Placentals and the Marsupials or Monotremes. Most certainly the differences are far greater than the differences in mental power between the dog, the ape, and man. Yet all these animals are genetically-related members of a single natural class.

[35] The English reader will recognise here the curious position of Dr. Wallace and of the late Dr. Mivart.—Translator.

We see this to a still more astonishing extent in the comparative psychology of another class of animals, that is especially interesting for many reasons—the insect class. It is well known that we find in many insects a degree of intelligence that is found in man alone among the Vertebrates. Everybody knows of the famous communities and states of bees and ants, and of the very remarkable social arrangements in them, such as we find among the more advanced races of men, but among no other group of animals. I need only mention the social organisation and government of the monarchic bees and the republican ants, and their division into different conditions—queen, drone-nobles, workers, educators, soldiers, etc. One of the most remarkable phenomena in this very interesting province is the cattle-keeping of the ants, which rear plant-lice as milch-cows and regularly extract their honeyed juice. Still more remarkable is the slave-holding of the large red ants, which steal the young of the small black ants and bring them up as slaves. It has long been known that these political and social arrangements of the ants are due to the deliberate cooperation of the countless citizens, and that they understand each other. A number of recent observers, especially Fritz Müller, Sir J. Lubbock (Lord Avebury), and August Forel, have put the astonishing degree of intelligence of these tiny Articulates beyond question.

Now, compare with these the mental life of many of the lower, especially the parasitic insects, as Darwin did. There is, for instance, the cochineal insect (Coccus), which, in its adult state, has a motionless, shield-shaped body, attached to the leaves of plants. Its feet are atrophied. Its snout is sunk in the tissue of the plants of which it absorbs the sap. The whole psychic life of these inert female parasites consists in the pleasure they experience from sucking the sap of the plant and in sexual intercourse with the males. It is the same with the maggot-like females of the fan-fly (Strepsitera), which spend their lives parasitically and immovably, without wings or feet, in the abdomen of wasps. There is no question here of higher psychic action. If we compare these sluggish parasites with the intelligent and active ants, we must admit that the psychic differences between them are much greater than the psychic differences between the lowest and highest mammals, between the Monotremes, Marsupials, and armadillos on the one hand, and the dog, ape, or man on the other. Yet all these insects belong to the same class of Articulates, just as all the mammals belong to one and the same class. And just as every consistent evolutionist must admit a common stem-form for all these insects, so he must also for all the mammals.

If we now turn from the comparative study of psychic life in different animals to the question of the organs of this function, we receive the answer that in all the higher animals they are always bound up with certain groups of cells, the ganglionic cells or neurona that compose the nervous system. All scientists without exception are agreed that the central nervous system is the organ of psychic life in the animal, and it is possible to prove this experimentally at any moment. When we partially or wholly destroy the central nervous system, we extinguish in the same proportion, partially or wholly, the “soul” or psychic activity of the animal. We have, therefore, to examine the features of the psychic organ in man. The reader already knows the incontestable answer to this question. Man’s psychic organ is, in structure and origin, just the same organ as in all the other Vertebrates. It originates in the shape of a simple medullary tube from the outer membrane of the embryo—the skin-sense layer. The simple cerebral vesicle that is formed by the expansion of the head-part of this medullary tube divides by transverse constrictions into five, and these pass through more or less the same stages of construction in the human embryo as in the rest of the mammals. As these are undoubtedly of a common origin, their brain and spinal cord must also have a common origin.

Physiology teaches us further, on the ground of observation and experiment, that the relation of the “soul” to its organ, the brain and spinal cord, is just the same in man as in the other mammals. The one cannot act at all without the other; it is just as much bound up with it as muscular movement is with the muscles. It can only develop in connection with it. If we are evolutionists at all, and grant the causal connection of ontogenesis and phylogenesis, we are forced to admit this thesis: The human soul or psyche, as a function of the medullary tube, has developed along with it; and just as brain and spinal cord now develop from the simple medullary tube in every human individual, so the human mind or the psychic life of the whole human race has been gradually evolved from the lower vertebrate soul. Just as to-day the intricate structure of the brain proceeds step by step from the same rudiment in every human individual—the same five cerebral vesicles—as in all the other Craniotes; so the human soul has been gradually developed in the course of millions of years from a long series of craniote-souls. Finally, just as to-day in every human embryo the various parts of the brain differentiate after the special type of the ape-brain, so the human psyche has proceeded historically from the ape-soul.

It is true that this Monistic conception is rejected with horror by most men, and the Dualistic idea, which denies the inseparable connection of brain and mind, and regards body and soul as two totally different things, is still popular. But how can we reconcile this view with the known facts of evolution? It meets with difficulties equally great and insuperable in embryology and in phylogeny. If we suppose with the majority of men that the soul is an independent entity, which has nothing to do with the body originally, but merely inhabits it for a time, and gives expression to its experiences through the brain just as the pianist does through his instrument, we must assign a point in human embryology at which the soul enters into the brain; and at death again we must assign a moment at which it abandons the body. As, further, each human individual has inherited certain personal features from each parent, we must suppose that in the act of conception pieces were detached from their souls and transferred to the embryo. A piece of the paternal soul goes with-the spermatozoon, and a piece of the mother’s soul remains in the ovum. At the moment of conception, when portions of the two nuclei of the copulating cells join together to form the nucleus of the stem-cell, the accompanying fragments of the immaterial souls must also be supposed to coalesce.

On this Dualistic view the phenomena of psychic development are totally incomprehensible. Everybody knows that the new-born child has no consciousness, no knowledge of itself and the surrounding world. Every parent who has impartially followed the mental development of his children will find it impossible to deny that it is a case of biological evolutionary processes. Just as all other functions of the body develop in connection with their organs, so the soul does in connection with the brain. This gradual unfolding of the soul of the child is, in fact, so wonderful and glorious a phenomenon that every mother or father who has eyes to observe is never tired of contemplating it. It is only our manuals of psychology that know nothing of this development; we are almost tempted to think sometimes that their authors can never have had children themselves. The human soul, as described in most of our psychological works, is merely the soul of a learned philosopher, who has read a good many books, but knows nothing of evolution, and never even reflects that his own soul has had a development.

When these Dualistic philosophers are consistent they must assign a moment in the phylogeny of the human soul at which it was first “introduced” into man’s vertebrate body. Hence, at the time when the human body was evolved from the anthropoid body of the ape (probably in the Tertiary period), a specific human psychic element—or, as people love to say, “a spark of divinity”—must have been suddenly infused or breathed into the anthropoid brain, and been associated with the ape-soul already present in it. I need not insist on the enormous theoretical difficulties of this idea. I will only point out that this “spark of divinity,” which is supposed to distinguish the soul of man from that of the other animals, must be itself capable of development, and has, as a matter of fact, progressively developed in the course of human history. As a rule, reason is taken to be this “spark of divinity,” and is supposed to be an exclusive possession of humanity. But comparative psychology shows us that it is quite impossible to set up this barrier between man and the brute. Either we take the word “reason” in the wider sense, and then it is found in the higher mammals (ape, dog, elephant, horse) just as well as in most men; or else in the narrower sense, and then it is lacking in most men just as much as in the majority of animals. On the whole, we may still say of man’s reason what Goethe’s Mephistopheles said:—

Life somewhat better might content him
But for the gleam of heavenly light that Thou hast given him.
He calls it reason; thence his power’s increased
To be still beastlier than any beast.

If, then, we must reject these popular and, in some respects, agreeable Dualistic theories as untenable, because inconsistent with the genetic facts, there remains only the opposite or Monistic conception, according to which the human soul is, like any other animal soul, a function of the central nervous system, and develops in inseparable connection therewith. We see this ontogenetically in every child. The biogenetic law compels us to affirm it phylogenetically. Just as in every human embryo the skin-sense layer gives rise to the medullary tube, from the anterior end of which the five cerebral vesicles of the Craniotes are developed, and from these the mammal brain (first with the characters of the lower, then with those of the higher mammals); and as the whole of this ontogenetic process is only a brief, hereditary reproduction of the same process in the phylogenesis of the Vertebrates; so the wonderful spiritual life of the human race through many thousands of years has been evolved step by step from the lowly psychic life of the lower Vertebrates, and the development of every child-soul is only a brief repetition of that long and complex phylogenetic process. From all these facts sound reason must conclude that the still prevalent belief in the immortality of the soul is an untenable superstition. I have shown its inconsistency with modern science in the eleventh chapter of The Riddle of the Universe.

Here it may also be well to point out the great importance of anthropogeny, in the light of the biogenetic law, for the purposes of philosophy. The speculative philosophers who take cognizance of these ontogenetic facts, and explain them (in accordance with the law) phylogenetically, will advance the great questions of philosophy far more than the most distinguished thinkers of all ages have yet succeeded in doing. Most certainly every clear and consistent thinker must derive from the facts of comparative anatomy and ontogeny we have adduced a number of suggestive ideas that cannot fail to have an influence on the progress of philosophy. Nor can it be doubted that the candid statement and impartial appreciation of these facts will lead to the decisive triumph of the philosophic tendency that we call “Monistic” or “Mechanical,” as opposed to the “Dualistic” or “Teleological,” on which most of the ancient, medieval, and modern systems of philosophy are based. The Monistic or Mechanical philosophy affirms that all the phenomena of human life and of the rest of nature are ruled by fixed and unalterable laws; that there is everywhere a necessary causal connection of phenomena; and that, therefore, the whole knowable universe is a harmonious unity, a monon. It says, further, that all phenomena are due solely to mechanical or efficient causes, not to final causes. It does not admit free-will in the ordinary sense of the word. In the light of the Monistic philosophy the phenomena that we are wont to regard as the freest and most independent, the expressions of the human will, are subject just as much to rigid laws as any other natural phenomenon. As a matter of fact, impartial and thorough examination of our “free” volitions shows that they are never really free, but always determined by antecedent factors that can be traced to either heredity or adaptation. We cannot, therefore, admit the conventional distinction between nature and spirit. There is spirit everywhere in nature, and we know of no spirit outside of nature. Hence, also, the common antithesis of natural science and mental or moral science is untenable. Every science, as such, is both natural and mental. That is a firm principle of Monism, which, on its religious side, we may also denominate Pantheism. Man is not above, but in, nature.

It is true that the opponents of evolution love to misrepresent the Monistic philosophy based on it as “Materialism,” and confuse the philosophic tendency of this name with a wholly unconnected and despicable moral materialism. Strictly speaking, it would be just as proper to call our system Spiritualism as Materialism. The real Materialistic philosophy affirms that the phenomena of life are, like all other phenomena, effects or products of matter. The opposite extreme, the Spiritualistic philosophy, says, on the contrary, that matter is a product of energy, and that all material forms are produced by free and independent forces. Thus, according to one-sided Materialism, the matter is antecedent to the living force; according to the equally one-sided view of the Spiritist, it is the reverse. Both views are Dualistic, and, in my opinion, both are false. For us the antithesis disappears in the Monistic philosophy, which knows neither matter without force nor force without matter. It is only necessary to reflect for some time over the question from the strictly scientific point of view to see that it is impossible to form a clear idea of either hypothesis. As Goethe said, “Matter can never exist or act without spirit, nor spirit without matter.”

The human “spirit” or “soul” is merely a force or form of energy, inseparably bound up with the material sub-stratum of the body. The thinking force of the mind is just as much connected with the structural elements of the brain as the motor force of the muscles with their structural elements. Our mental powers are functions of the brain as much as any other force is a function of a material body. We know of no matter that is devoid of force, and no forces that are not bound up with matter. When the forces enter into the phenomenon as movements we call them living or active forces; when they are in a state of rest or equilibrium we call them latent or potential. This applies equally to inorganic and organic bodies. The magnet that attracts iron filings, the powder that explodes, the steam that drives the locomotive, are living inorganics; they act by living force as much as the sensitive Mimosa does when it contracts its leaves at touch, or the venerable Amphioxus that buries itself in the sand of the sea, or man when he thinks. Only in the latter cases the combinations of the different forces that appear as “movement” in the phenomenon are much more intricate and difficult to analyse than in the former.

Our study has led us to the conclusion that in the whole evolution of man, in his embryology and in his phylogeny, there are no living forces at work other than those of the rest of organic and inorganic nature. All the forces that are operative in it could be reduced in the ultimate analysis to growth, the fundamental evolutionary function that brings about the forms of both the organic and the inorganic. But growth itself depends on the attraction and repulsion of homogeneous and heterogeneous particles. Seventy-five years ago Carl Ernst von Baer summed up the general result of his classic studies of animal development in the sentence: “The evolution of the individual is the history of the growth of individuality in every respect.” And if we go deeper to the root of this law of growth, we find that in the long run it can always be reduced to that attraction and repulsion of animated atoms which Empedocles called the “love and hatred” of the elements.

Thus the evolution of man is directed by the same “eternal, iron laws” as the development of any other body. These laws always lead us back to the same simple principles, the elementary principles of physics and chemistry. The various phenomena of nature only differ in the degree of complexity in which the different forces work together. Each single process of adaptation and heredity in the stem-history of our ancestors is in itself a very complex physiological phenomenon. Far more intricate are the processes of human embryology; in these are condensed and comprised thousands of the phylogenetic processes.

In my General Morphology, which appeared in 1866, I made the first attempt to apply the theory of evolution, as reformed by Darwin, to the whole province of biology, and especially to provide with its assistance a mechanical foundation for the science of organic forms. The intimate relations that exist between all parts of organic science, especially the direct causal nexus between the two sections of evolution—ontogeny and phylogeny—were explained in that work for the first time by transformism, and were interpreted philosophically in the light of the theory of descent. The anthropological part of the General Morphology (Book vii) contains the first attempt to determine the series of man’s ancestors (vol. ii, p. 428). However imperfect this attempt was, it provided a starting-point for further investigation. In the thirty-seven years that have since elapsed the biological horizon has been enormously widened; our empirical acquisitions in paleontology, comparative anatomy, and ontogeny have grown to an astonishing extent, thanks to the united efforts of a number of able workers and the employment of better methods. Many important biological questions that then appeared to be obscure enigmas seem to be entirely settled. Darwinism arose like the dawn of a new day of clear Monistic science after the dark night of mystic dogmatism, and we can say now, proudly and gladly, that there is daylight in our field of inquiry.

Philosophers and others, who are equally ignorant of the empirical sources of our evidence and the phylogenetic methods of utilising it, have even lately claimed that in the matter of constructing our genealogical tree nothing more has been done than the discovery of a “gallery of ancestors,” such as we find in the mansions of the nobility. This would be quite true if the genealogy given in the second part of this work were merely the juxtaposition of a series of animal forms, of which we gathered the genetic connection from their external physiognomic resemblances. As we have sufficiently proved already, it is for us a question of a totally different thing—of the morphological and historical proof of the phylogenetic connection of these ancestors on the basis of their identity in internal structure and embryonic development; and I think I have sufficiently shown in the first part of this work how far this is calculated to reveal to us their inner nature and its historical development. I see the essence of its significance precisely in the proof of historical connection. I am one of those scientists who believe in a real “natural history,” and who think as much of an historical knowledge of the past as of an exact investigation of the present. The incalculable value of the historical consciousness cannot be sufficiently emphasised at a time when historical research is ignored and neglected, and when an “exact” school, as dogmatic as it is narrow, would substitute for it physical experiments and mathematical formulæ. Historical knowledge cannot be replaced by any other branch of science.

It is clear that the prejudices that stand in the way of a general recognition of this “natural anthropogeny” are still very great; otherwise the long struggle of philosophic systems would have ended in favour of Monism. But we may confidently expect that a more general acquaintance with the genetic facts will gradually destroy these prejudices, and lead to the triumph of the natural conception of “man’s place in nature.” When we hear it said, in face of this expectation, that this would lead to retrogression in the intellectual and moral development of mankind, I cannot refrain from saying that, in my opinion, it will be just the reverse; that it will promote to an enormous extent the advance of the human mind. All progress in our knowledge of truth means an advance in the higher cultivation of the human intelligence; and all progress in its application to practical life implies a corresponding improvement of morality. The worst enemies of the human race—ignorance and superstition—can only be vanquished by truth and reason. In any case, I hope and desire to have convinced the reader of these chapters that the true scientific comprehension of the human frame can only be attained in the way that we recognise to be the sole sound and effective one in organic science generally—namely, the way of Evolution.

INDEX

A
Abiogenesis, [26]
Accipenser, [234]
Abortive ova, [55]
Achromatin, [42]
Achromin, [42]
Acœla, [221]
Acoustic nerve, the, [289,] [290]
Acquired characters, inheritance of, [349]
Acrania, the, [182]
Acroganglion, the, [268,] [275]
Adam’s apple, the, [184]
Adapida, [257]
Adaptation, [3,] [5,] [27]
After-birth, the, [167]
Agassiz, L., [34]
Age of life, [200]
Alimentary canal, evolution of the, [13,] [14,] [133,] [308–17]
— — structure of the, [169,] [308–10]
Allantoic circulation, the, [171]
Allantois, development of the, [166]
Allmann, [20]
Amblystoma, [243]
Amitotic cleavage, [40]
Ammoconida, [217]
Ammolynthus, [217]
Amnion, the, [115]
— formation of the, [134,] [244]
Amniotic fluid, the, [134]
Amœba, the, [47–9,] [210]
Amphibia, the, [239]
Amphichœrus, [221]
Amphigastrula, [80]
Amphioxus, the, [105,] [181–95]
— circulation of the, [184]
— cœlomation of the, [95]
— embryology of the, [191–95]
— structure of the, [183–88]
Amphirhina, [230]
Anamnia, the, [115]
Anatomy, comparative, [208]
Animalculists, [12]
Animal layer, the, [16]
Annelids, the, [142,] [219]
Annelid theory, the, [142]
Anomodontia, [246]
Ant, intelligence of the, [353]
Anthropithecus, [174,] [262]
Anthropogeny, [1]
Anthropoid apes, the, [166,] [173,] [262]
Anthropology, [1,] [35]
Anthropozoic period, [203]
Antimera, [107]
Anura, [243]
Anus, the, [317]
Anus, formation of the, [139]
Aorta, the, [327]
— development of the, [170]
Ape and man, [157,] [164,] [261,] [307,] [351]
Ape-man, the, [263]
Apes, the, [257–60]
Aphanocapsa, [210]
Aphanostomum, [221]
Appendicaria, [197]
Appendix vermiformis, the, [32]
Aquatic life, early prevalence of, [235]
Ararat, Mount, [24]
Archenteron, [64,] [74]
Archeolithic age, [203]
Archicaryon, [55]
Archicrania, [230]
Archigastrula, [65,] [193]
Archiprimas, [263]
Arctopitheca, [261]
Area, the germinative, [121]
Aristotle, [9]
Arm, structure of the, [306]
Arrow-worm, the, [191]
Arterial arches, the, [325–26]
— cone, the, [324]
Arteries, evolution of the, [170,] [323–24]
Articulates, the, [142,] [219]
— skeleton of the, [294]
Articulation, [141–42]
Aryo-Romanic languages, the, [203]
Ascidia, the, [181,] [188–90]
— embryology of the, [196–98]
Ascula, [217]
Asexual reproduction, [51]
Atlas, the, [247]
Atrium, the, [183,] [185]
— (heart), the, [326]
Auditory nerve, the, [289,] [290]
Auricles of the heart, [325]
Autolemures, [257]
Axolotl, the, [243]
B
Bacteria, [38,] [210]
Baer, K. E. von, [15–17]
Balanoglossus, [226]
Balfour, F., [21]
Batrachia, [241]
Bdellostoma Stouti, [78]
Bee, generation of the, [9]
Beyschlag, W., on evolution, [50]
Bilateral symmetry, [66]
— — origin of, [221]
Bimana, [258]
Biogenetic law, the, [2,] [21,] [23,] [179,] [349]
Biogeny, [2]
Bionomy, [33]
Bird, evolution of the, [245]
— ovum of the, [44–6,] [80–1]
Bischoff, W., [17]
Bladder, evolution of the, [244,] [339]
Blastæa, the, [206,] [213]
Blastocœl, the, [62,] [74]
Blastocrene, the, [99]
Blastocystis, the, [62,] [119,] [120]
Blastoderm, the, [62]
Blastodermic vesicle, the, [119]
Blastoporus, the, [64]
Blastosphere, the, [62,] [119]
Blastula, the, [62,] [74]
— the mammal, [119]
Blood, importance of the, [318]
— recent experiments in mixture of, [172]
— structure of the, [319]
Blood-cells, the, [319]
Blood-vessels, the, [318–25]
— development of the, [168]
— of the vertebrate, [110]
— origin of the, [320–21]
Boniface VIII, Bull of, [10]
Bonnet, [13]
Borneo nosed-ape, the, [164]
Boveri, Theodor, [185]
Brachytarsi, [257]
Brain and mind, [278,] [354–56]
— evolution of the, [8,] [275–80]
— in the fish, [276]
— in the lower animals, [275]
— structure of the, [273–74]
Branchial arches, evolution of the, [303]
— cavity, the, [183,] [189]
— system, the, [110]
Branchiotomes, [149]
Breasts, the, [113]
Bulbilla, [184]
C
Calamichthys, [234]
Calcolynthus, [217]
Capillaries, the, [323]
Caracoideum, the, [249]
Carboniferous strata, [202]
Carcharodon, [234]
Cardiac cavity, the, [170]
Cardiocœl, the, [328]
Caryobasis, [38,] [54]
Caryokinesis, [42]
Caryolymph, [38,] [54]
Caryolyses, [42]
Caryon, [37]
Caryoplasm, [37]
Catallacta, [213]
Catarrhinæ, the, [173,] [261]
Catastrophic theory, the, [24]
Caudate cells, [53]
Cell, life of the, [41–3]
— nature of the, [36–7]
— size of the, [38]
Cell theory, the, [18,] [36]
Cenogenesis, [4]
Cenogenetic structures, [4]
Cenozoic period, the, [203]
Central body, the, [38,] [42]
Central nervous system, the, [273]
Centrolecithal ova, [68]
Centrosoma, the, [38,] [42]
Ceratodus, the, [76,] [237]
Cerebellum, the, [274]
Cerebral vesicles, evolution of the, [276]
Cerebrum, the, [273]
Cestracion Japonicus, [75,] [79]
Chætognatha, [94]
Chick, importance of the, in embryology, [11,] [16]
Child, mind of the, [8,] [355]
Chimpanzee, the, [174,] [262]
Chiromys, [257]
Chiroptera, [258]
Chirotherium, [239]
Chondylarthra, [257]
Chorda, the, [17,] [95,] [107,] [183]
— evolution of the, [296]
Chordæa, the, [97]
Chordalemma, the, [296]
Chordaria, [97]
Chordula, the, [3,] [96,] [191]
Choriata, the, [166]
Chorion, the, [119]
— development of the, [165–6]
— frondosum, [255]
— læve, [255]
Choroid coat, the, [286]
Chorology, [33]
Chromacea, [209]
Chromatin, [42]
Chroococcacea, [210]
Chroococcus, the, [210]
Church, opposition of, to science in Middle Ages, [10]
Chyle, [318]
Chyle-vessels, [324]
Cicatricula, the, [45,] [81]
Ciliated cells, [53,] [193]
Cinghalese gynecomast, [114]
Circulation in the lancelet, [184]
Circulatory system, evolution of the, [321–25]
— — structure of the, [318]
Classification, [103]
— evolutionary value of, [33]
Clitoris, the, [345]
Cloaca, the, [249,] [317]
Cnidaria, [217]
Coccyx, the, [295]
Cochineal insect, the, [354]
Cochlea, the, [289]
Cœcilia, [241]
Cœcum, the, [310,] [317]
Cœlenterata, [20,] [91,] [93,] [104]
Cœlenteria, [221]
Cœloma, the, [21,] [64,] [91]
Cœlomæa, the, [98]
Cœlomaria, [21,] [91,] [104,] [221]
Cœlomation, [93–4]
Cœlom-theory, the, [21,] [93]
Cœlomula, the, [98]
Colon, the, [310,] [317]
Comparative anatomy, [31]
Conception, nature of, [51]
Conjunctiva, the, [286]
Conocyema, [215]
Convoluta, [221]
Copelata, the, [197]
Copulative organs, evolution of the, [344–45]
Corium, the, [108,] [268]
Cornea, the, [286]
Corpora cavernosa, the, [345,] [346]
Corpora quadrigemina, [274]
Corpora striata, [274]
Corpus callosum, the, [274]
Corpus vitreum, the, [285]
Corpuscles of the blood, [319]
Craniology, [303]
Craniota, the, [182,] [229]
Cranium, the, [299]
Creation, [23–4]
Cretaceous strata, [202]
Crossopterygii, [234]
Crustacea, the, [142,] [219]
Cryptocœla, [221]
Cryptorchism, [114]
Crystalline lens, the, [285]
— — development of the, [287]
Cutaneous glands, [268]
Cuttlefish, embryology of the, [9]
Cuvier, G., [17,] [24]
Cyanophycea, [209]
Cyclostoma, the, [188,] [230–32]
— ova of the, [75]
Cyemaria, [214]
Cynopitheca, [262]
Cynthia, [191,] [196]
Cytoblastus, the, [37]
Cytodes, [40]
Cytoplasm, [37,] [38]
Cytosoma, [37]
Cytula, the, [54]
D
Dalton, [15]
Darwin, C., [2,] [5,] [23,] [28–9]
Darwin, E., [28]
Darwinism, [5,] [28]
Decidua, the, [167]
Deciduata, [255]
Deduction, nature of, [208]
Degeneration theory, the, [219]
Dentition of the ape and man, [259]
Depula, [62]
Descent of Man, [30]
Design in organisms, [33]
Deutoplasm, [44]
Devonian strata, [202]
Diaphragm, the, [309]
— evolution of the, [328]
Dicyema, [215]
Dicyemida, [215]
Didelphia, [248]
Digonopora, [223]
Dinosauria, [202]
Dipneumones, [238]
Dipneusta, [235–38]
— ova of the, [75]
Dipnoa, [236]
Directive bodies, [54]
Discoblastic ova, [68]
Discoplacenta, [255]
Dissatyrus, [174]
Dissection, medieval decrees against, [10]
Dohrn, Anton, [219]
Döllinger, [15]
Dorsal furrow, the, [125]
— shield, the, [123]
— zone, the, [129]
Dromatherium, [248]
Dualism, [6]
Dubois, Eugen, [263]
Ductus Botalli, the, [350]
Ductus venosus Arantii, [350]
Duodenum, the, [309,] [317]
Duration of embryonic development, [199]
— of man’s history, [199]
Dysteleology, [32]
— proofs of, [349]
E
Ear, evolution of the, [288–92]
— structure of the, [288]
— uselessness of the external, [32]
Ear-bones, the, [289]
Earth, age of the, [200–201]
Echidna hystrix, [249]
Ectoblast, [20,] [64]
Ectoderm, the, [20,] [64]
Edentata, [250]
Efficient causes, [6]
Egg of the bird, [44–6,] [81]
— or the chick, priority of the, [211]
Elasmobranchs, the, [79]
Embryo, human, development of the, [158]
Embryology, [2]
— evolutionary value of, [34]
Embryonic development, duration of, [199]
— disk, the, [121–22]
— spot, the, [125]
Encephalon, the, [273]
Endoblast, [20,] [64]
Endothelia, [321]
Enterocœla, [93,] [223]
Enteropneusta, [226]
Entoderm, the, [20,] [64]
Eocene strata, [203]
Eopitheca, [259]
Epiblast, [20,] [64]
Epidermis, the, [108,] [268]
Epididymis, the, [342]
Epigastrula, [80]
Epigenesis, [11,] [13]
Epiglottis, the, [309]
Epiphysis, the, [108]
Episoma, [129]
Episomites, [130,] [194]
Epispadia, [346]
Epithelia, [37]
Epitheria, [243,] [253]
Epovarium, the, [342]
Equilibrium, sense of, [291]
Esthonychida, [257]
Eustachian tube, the, [289]
Eutheria, [253]
Eve, [12]
Evolution theory, the, [11,] [208]
— inductive nature of, [30]
Eye, evolution of the, [285–88]
— structure of the, [285]
Eyelid, the third, [32]
Eyelids, evolution of the, [288]
F
Fabricius ab Aquapendente, [10]
Face, embryonic development of the, [284]
Fat glands in the skin, [269]
Feathers, evolution of, [270]
Fertilisation, [51]
— place of, [119]
Fin, evolution of the, [239,] [304]
Final causes, [6]
Flagellate cells, [193]
Floating bladder, the, [233,] [241]
— — evolution of the, [314]
Fœtal circulation, [170–71]
Food-yelk, the, [67,]
Foot, evolution of the, [241,] [304–6]
— of the ape and man, [258–59]
Fore brain, the, [278]
Fore kidneys, the, [336,] [337]
Fossiliferous strata, list of, [201]
Fossils, [180]
— scarcity of, [208]
Free will, [356]
Friedenthal, experiments of, [172]
Frog, the, [241–42]
— ova of the, [71–2]
Frontonia, [224]
Function and structure, [7]
Furcation of ova, [72]
G
Gaertner’s duct, [341,] [350]
Ganglia, commencement of, [268]
Ganglionic cell, the, [39]
Ganoids, [233,] [234]
Gastræa, the,

[3,] [20,] [206]
— formation of the, [213]
Gastræa theory, the, [20,] [64,] [69]
Gastræads, [69,] [214]
Gastremaria, [214]
Gastrocystis, the, [62,] [119,] [120]
Gastrophysema, [215]
Gastrotricha, [224]
Gastrula, the, [3,] [20,] [62]
Gastrulation, [62]
Gegenbaur, Carl, [220]
— on evolution, [32]
— on the skull, [300–1]
Gemmation, [331]
General Morphology, [8,] [29]
Genesis, [23]
Genital pore, the, [335]
Geological evolution, length of, [200]
— periods, [201]
Geology, methods of, [180]
— rise of, [24]
Germ-plasm, theory of, [349]
Germinal disk, [46,] [81]
— layers, the, [14,] [16]
— — scheme of the, [92]
— spot, the, [44]
— vesicle, the, [43,] [54]
Germinative area, the, [121]
Giant gorilla, the, [176]
Gibbon, the, [173,] [262]
Gill-clefts and arches, [110]
— formation of the, [151–52,] [303]
Gill-crate, the, [183,] [189]
Gills, disappearance of the, [244]
Glœocapsa, [210]
Gnathostoma, [230,] [232]
Goethe as an evolutionist, [27,] [299]
Goitre, [110]
Gonads, the, [111]
— formation of the, [149–50]
Gonidia, [334]
Gonochorism, beginning of, [322]
Gonoducts, [335]
Gonotomes, [146,] [149]
Goodsir, [189]
Gorilla, the, [174,] [176,] [262]
Graafian follicles, the, [17,] [119,] [347]
Gregarinæ, [211]
Gullet-ganglion, the, [190]
Gut, evolution of the, [310–17]
Gyrini, [242]
Gynecomastism, [114]
H
Hag-fish, the, [188]
Hair, evolution of the, [270]
— on the human embryo and infant, [271]
Hair, restriction of, by sexual selection, [271]
Haliphysema, [215]
Halisauria, [202]
Haller, Albrecht, [12]
Halosphæra viridis, [213]
Hand, evolution of the, [250,] [304–6]
— of the ape and man, [258]
Hapalidæ, [261]
Harderian gland, the, [288]
Hare-lip, [284]
Harrison, Granville, [161]
Hartmann, [262]
Harvey, [10]
Hatschek, [192]
Hatteria, [243,] [246]
Head-cavity, the, [138]
Head-plates, the, [149]
Heart, development of the, [7,] [10,] [111,] [151,] [170,] [322,] [324–27]
— of the ascidia, [190]
— position of the, [327]
Helmholtz, [207]
Helminthes, [223]
Hepatic gut, the, [109,] [316]
Heredity, nature of, [3,] [5,] [27,] [56–7,] [349]
Hermaphrodism, [9,] [23,] [114,] [218,] [322,] [346]
Hertwig, [21]
Hesperopitheca, [259]
His, W., [19]
Histogeny, [18,] [19]
History of Creation, [6,] [30]
Holoblastic ova, [67,] [71,] [77]
Homœosaurus, [244,] [246]
Homology of the germinal layers, [20]
Hoof, evolution of the, [270]
Hunterian ligament, the, [344]
Huxleian law, the, [171,] [257,] [262]
Huxley, T. H., [7,] [20,] [29]
Hydra, the, [69,] [217]
Hydrostatic apparatus in the fish, [315]
Hylobates, [173,] [262]
Hylodes Martinicensis, [241]
Hyoid bone, the, [299]
Hypermastism, [113]
Hyperthelism, [113]
Hypoblast, [20,] [64]
Hypobranchial groove, the, [110,] [184,] [226,] [316]
Hypodermis, the, [268]
Hypopsodina, [257]
Hyposoma, the, [129]
Hyposomites, [130,] [194]
Hypospadia, [346]
I
Ichthydina, [224]
Ichthyophis glutinosa, [80]
Ictopsida, [257]
Ileum, the, [310]
Immortality, Aristotle on, [10]
Immortality of the soul, [58]
Impregnation-rise, the, [55]
Indecidua, [255]
Indo-Germanic languages, [203]
Induction and deduction, [31,] [208]
Inheritance of acquired characters, [349]
Insects, intelligence of, [353]
Interamniotic cavity, the, [165]
Intestines, the, [309,] [316–17]
Invagination, [62]
Iris, the, [286]
J
Jacchus, [261]
Java, ape-man of, [263,] [264]
Jaws, evolution of the, [301]
Jurassic strata, [202]
K
Kant, dualism of, [25]
Kelvin, Lord, on the origin of life, [207]
Kidneys, the, [111]
— formation of the, [150–51,] [336–42]
Klaatsch, [262]
Kölliker, [21]
Kowalevsky, [191]
L
Labia, the, [346]
Labyrinth, the, [290]
Lachrymal glands, [269]
Lamarck, J., [23,] [25–7]
— theories of, [26,] [349]
Lamprey, the, [230]
— ova of the, [75]
Lancelet, the, [60,] [181–95]
— description of the, [105]
Languages, evolution of, [203]
Lanugo of the embryo, [271]
Larynx, the, [309]
— evolution of the, [314]
Latebra, the, [45]
Lateral plates, the, [129]
Laurentian strata, [201]
Lecithoma, the, [117]
Leg, evolution of the, [304]
— structure of the, [306]
Lemuravida, [257]
Lemurogona, [257]
Lemurs, the, [257]
Lepidosiren, [257]
Leucocytes, [319]
Life, age of, [200]
Limbs, evolution of the, [152,] [239,] [304]
Limiting furrow, the, [133]
Linin, [42]
Liver, the, [309,] [317]
Long-nosed ape, the, [164]
Love, importance of in nature, [332]
Lungs, the, [110]
— evolution of the, [241,] [314–15]
Lyell, Sir C., [24]
Lymphatic vessels, the, [318]
Lymph-cells, the, [319]
M
Macrogonidion, [331]
Macrospores, [331]
Magosphæra planula, [213]
Male womb, the, [344,] [350]
Mallochorion, the, [166]
Mallotheria, [257]
Malpighian capsules, [339,] [341]
Mammal, characters of the, [112]
— gastrulation of the, [84]
Mammals, unity of the, [247–48]
Mammary glands, the, [113,] [269]
Man and the ape, relation of, [262,] [351]
— origin of, [29]
Man’s Place in Nature, [7,] [29,] [351]
Mantle, the, [189]
Mantle-folds, the, [185]
Marsupials, the, [250–52]
— ova of the, [85]
Materialism, [356]
Mathematical method, the, [30]
Mechanical causes, [6]
— embryology, [8,] [19,] [22]
Meckel’s cartilage, [304]
Medulla capitis, the, [273]
— oblongata, the, [274]
— spinalis, the, [273]
Medullary groove, the, [125]
— tube, the, [107,] [128]
— — formation of the, [131,] [133,] [227,] [267,] [276]
Mehnert, E., on the biogenetic law, [5]
Meroblastic ova, [67,] [71,] [78]
Merocytes, [68,] [321]
Mesentery, the, [98,] [109,] [310,] [316]
Mesocardium, the, [327]
Mesoderm, the, [20,] [64,] [90,] [93]
Mesogastria, [215]
Mesonephridia, the, [338]
Mesonephros, the, [336]
Mesorchium, the, [344]
Mesovarium, the, [344]
Mesozoic period, the, [202]
Metogaster, the, [64]
Metagastrula, the, [67]
Metamerism, [142]
Metanephridia, the, [341]
Metanephros, the, [336]
Metaplasm, [39]
Metastoma, [64,] [222]
Metatheria, [248]
Metazoa, [20,] [62]
Metovum, the, [81]
Microgonidian, [331]
Microspores, [331]
Middle ear, the, [291]
Migration, effect of, [33]
Milk, secretion of the, [269]
Mind, evolution of, [353–54]
— in the lower animals, [353]
Miocene strata, [203]
Mitosis, [40,] [41]
Monera, [40,] [206,] [209]
Monism, [6,] [356]
Monodelphia, [248]
Monogonopora, [223]
Monopneumones, [238]
Monotremes, [118,] [249]
— ova of the, [84]
Monoxenia Darwinii, [60]
Morea, the, [212]
Morphology, [2,] [27]
Morula, the, [62,] [212]
Motor-germinative layer, the, [19]
Mouth, development of the, [124,] [139]
— structure of the, [308]
Mucous layer, the, [16]
Müllerian duct, the, [341]
Muscle-layer, the, [16]
Muscles, evolution of the, [307]
— of the ear, rudimentary, [292]
Myotomes, [108,] [146]
Myxinoides, the, [188,] [230]
N
Nails, evolution of the, [270]
Nasal pits, [284]
Natural philosophy, [25]
— selection, [26,] [28,] [349]
Navel, the, [117,] [134]
Necrolemurs, [257]
Nectocystis, the, [314]
Nemertina, [224–26]
Nephroduct, evolution of the, [338–39]
Nephrotomes, [149,] [338]
Nerve-cell, the, [39]
Nerves, animals without, [267]
Nervous system, evolution of the, [7,] [267]
Neurenteric canal, the, [127]
Nictitating membrane, the, [32,] [286,] [288]
Nose, the, in man and the ape, [164]
— development of the, [282–85]
— structure of the, [283]
Notochorda, the, [107]
Nuclein, [37]
Nucleolinus, [44]
Nucleolus, the, [38,] [44,] [54]
Nucleus of the cell, [37]
O
Œsophagus, the, [309,] [316]
Oken, [5,] [27,] [300]
Oken’s bodies, [339]
Oligocene strata, [203]
Olynthus, [217]
On the generation of animals, [9]
Ontogeny, [2,] [23]
— defective evidence of, [208]
Opaque area, the, [122]
Opossum, the, [252]
— ova of the, [85]
Optic nerve, the, [287]
Optic thalami, [274]
— vesicles, [286]
Orang, the, [174,] [262]
Ornithodelphia, [248]
Ornithorhyncus, [85,] [249]
Ornithostoma, [249]
Ossicles of the ear, [289]
Otoliths, [289]
Ova, number of, [347]
— of the lancelet, [192]
Ovaries, evolution of the, [333–34]
Oviduct, origin of the, [335,] [342]
Ovolemma, the, [44]
Ovulists, [12]
Ovum, discovery of the, [16]
— nature of the, [40
— size of the, ][44]
P
Pachylemurs, the, [257]
Pacinian corpuscles, [282]
Paleontology, [2]
— evolutionary evidence of, [31]
— incompleteness of, [208]
— rise of, [24]
Paleozoic age, the, [202]
Palingenesis, [4]
Palingenetic structures, [4]
Palæhatteria, [244,] [246]
Panniculus carnosus, the, [350]
Paradidymis, the, [342]
Parietal zone, the, [129]
Parthenogenesis, [9,] [13]
Pastrana, Miss Julia, [164]
Pedimana, [252]
Pellucid area, the, [122]
Pelvic cavity, the, [138]
Pemmatodiscus gastrulaceus, [215]
Penis-bone, the, [346]
Penis, varieties of the, [345]
Peramelida, [254]
Periblastic ova, [68]
Peribranchial cavity, the, [185,] [190]
Pericardial cavity, the, [328]
Perichorda, the, [108,] [183]
— formation of the, [136]
Perigastrula, [89]
Permian strata, [202]
Petromyzontes, the, [188,] [230]
Phagocytes, [49,] [320]
Pharyngeal ganglion, the, [275]
Pharynx, the, [309]
Philology, comparison with, [203]
Philosophie Zoologique, [25]
Philosophy and evolution, [6]
Phycochromacea, [209]
Phylogeny, [2,] [23]
Physemaria, [214]
Physiology, backwardness of, [7]
Phytomonera, [209]
Pineal eye, the, [108]
Pinna, the, [291]
Pithecanthropus, [263,] [264]
Pithecometra-principle, the, [171]
Placenta, the, [166,] [253–54]
Placentals, the, [166]
— characters of the, [253]
— gastrulation of the, [86]
Planocytes, [49,] [320]
Plant-louse, parthenogenesis of the, [13]
Planula, the, [89]
Plasma-products, [38,] [39]
Plasson, [40,] [59]
Plastids, [36,] [40,] [209]
Plastidules, [59]
Platodaria, [221]
Platodes, the, [221]
Platyrrhinæ, [261]
Pleuracanthida, [234]
Pleural ducts, [328]
Pliocene strata, [203]
Polar cells, [54]
Polyspermism, [58]
Preformation theory, the, [11]
Primary period, the, [202]
Primates, the, [157,] [257–60]
Primatoid, [263]
Primitive groove, the, [69,] [82,] [124,] [125]
— gut, the, [20,] [63,] [214]
— kidneys, the, [111,] [337]
— mouth, the, [20,] [63]

— segments, [143]
— streak, the, [100,] [122]
— vertebræ, [144,] [195,] [206,] [229]
Primordial period, the, [201]
Prochordata, [192]
Prochordonia, the, [192,] [218]
Prochoriata, [253]
Prochorion, the, [44,] [119]
Proctodæum, the, [345]
Procytella primordialis, [210]
Prodidelphia, [256]
Progaster, the, [20,] [63]
Progonidia, [333]
Promammalia, [247]
Pronephridia, the, [151]
Pronucleus femininus, [54]
— masculinus, [54]
Properistoma, [69]
Prorenal canals of the lancelet, [186]
— duct, the, [132,] [139,] [186]
— — evolution of the, [338]
Proselachii, [234]
Prosimiæ, the, [257]
Prospermaria, [333]
Prospondylus, [105,] [229]
Prostoma, [20,] [63,] [222]
Protamniotes, [243–44]
Protamœba, [210]
Proterosaurus, the, [202,] [244]
Protists, [36,] [38]
Protonephros, [111,] [336]
Protophyta, [210]
Protoplasm, [37,] [209]
Protopterus, [238]
Prototheria, [248]
Protovertebræ, [142,] [144]
Protozoa, [20,] [210]
Provertebral cavity, the, [148]
— plates, the, [136,] [144]
Pseudocœla, [93,] [221]
Pseudopodia, [48]
Pseudova, [13]
Psychic life, evolution of the, [8]
Psychology, [8]
Pterosauria, [202]
Pylorus, the, [309]
Q
Quadratum, the, [247]
Quadrumana, [258]
Quaternary period, [203]
R
Rabbit, ova of the, [86–7]
Radiates, the, [103]
Rathke’s canals, [341]
Rectum, the, [317]
Regner de Graaf, [119]
Renal system, evolution of the, [335–42]
Reproduction, nature of, [330–31]
Reptiles, [245–47]
Respiratory organs, evolution of the, [314–15]
— pore, the, [183,] [189]
Retina, the, [286]
Rhabdocœla, [222]
Rhodocytes, [319]
Rhopalura, [215]
Rhyncocephala, [243]
Ribs, the, [295]
— number of the, [353]
Rudimentary ear-muscles, [292]
— organs, [32]
— — list of, [349–50]
— toes, [306]
S
Sacculus, the, [289]
Sagitta, [65,] [66,] [191]
— cœlomation of, [93]
Salamander, the, [241]
— ova of the, [74]
Sandal-shape of embryo, [128–29]
Satyrus, [174,] [262]
Sauromammalia, [246]
Sauropsida, [245]
Scatulation theory, the, [12]
Schizomycetes, [210]
Schleiden, M., [18,] [36]
Schwann, T., [18,] [36]
Sclerotic coat, the, [286]
Sclerotomes, [108,] [143,] [148]
Scrotum, the, [344]
Scyllium, nose of the, [283]
Sea-squirt, the, [181,] [188–90]
Secondary period, the, [202]
Segmentation, [60,] [141–42]
Segmentation-cells, [54]
Segmentation-sphere, the, [17]
Selachii, [223]
— skull of the, [301]
Selection, theory of, [28]
Selenka, [166,] [168]
Semnopitheci, [262]
Sense-organs, evolution of the, [151,] [280]
— number of the, [281]
— origin of the, [281]
Sensory nerves, [279]
Serocœlom, the, [165]
Serous layer, the, [16]
Sex-organs, early vertebrate form of the, [111]
— evolution of the, [333]
Sexual reproduction, simplest forms of, [331]
— selection, [30,] [271–72]
Shark, the, [233]
— nose of the, [283]
— ova of the, [75]
— placenta of the, [9]
— skull of the, [301]
Shoulder-blade, the, [306]
Sickle-groove, the, [82,] [121]
Sieve-membrane, the, [167]
Silurian strata, [202]
Simiæ, the, [257–60]
Siphonophoræ, embryology of the, [21]
Skeleton, structure of the, [294]
Skeleton-plate, the, [148]
Skin, the, [151]
— evolution of, [266–69]
— function of the, [269]
Skin-layer, the, [16]
Skull, evolution of the, [149,] [299–303]
— structure of the, [299]
— vertebral theory of the, [300]
Smell, the sense of, [282]
Soul, evolution of the, [353–56]
— nature of the, [58,] [356]
— phylogeny of the, [8]
— seat of the, [278]
Sound, sensations of, [289–90]
Sozobranchia, [242]
Space, sense of, [291]
Species, nature of the, [23,] [34]
Speech, evolution of, [264]
Spermaducts, [335,] [342]
Spermaries, evolution of the, [333–34]
Spermatozoon, the, [52–3]
— discovery of the, [12,] [53]
Spinal cord, development of the, [8]
— structure of the, [273]
Spirema, the, [42]
Spiritualism, [356]
Spleen, the, [318]
Spondyli, [142]
Sponges, classification of the, [34]
— ova of the, [49]
Spontaneous generation, [26,] [206]
Stegocephala, [239]
Stem-cell, the, [54]
Stem-zone, the, [129]
Stomach, evolution of the, [311–14,] [316]
— structure of the human, [309]
Strata, thickness of, [200–201]
Struggle for life, the, [28]
Subcutis, the, [268]
Sweat glands, [269]
T
Tactile corpuscles, [268,] [282]
Tadpole, the, [242]
Tail, evolution of the, [242–43]
— rudimentary, in man, [159,] [295,] [350]
Tailed men, [160–61]
Taste, the sense of, [282]
Teeth, evolution of the, [314]
— of the ape and man, [259]
Teleostei, [234]
Telolecithal ova, [67,] [68]
Temperature, sense of, [282]
Terrestrial life, beginning of, [235]
Tertiary period, the, [203]
Theoria generationis, the, [13]
Theories, value of, [181]
Theromorpha, [246]
Third eyelid, the, [286,] [288]
Thyroid gland, the, [110,] [184,] [315]
Time-variations in ontogeny, [5]
Tissues, primary and secondary, [37]
Toad, the, [241]
Tocosauria, [246]
Toes, number of the, [240]
Tori genitales, the, [346]
Touch, the sense of, [282]
Tracheata, [142,] [219]
Tread, the, [45,] [81]
Tree-frog, the, [241]
Triassic strata, [202]
Triton tæniatus, [74]
Troglodytes, [174]
Tunicates, the, [189]
Turbellaria, [222]
Turbinated bones, the, [283]
Tympanic cavity, the, [288]
U
Umbilical, cord, the, [117]
— vesicle, the, [138]
Unicellular ancestor of all animals, [47]
— animals, [38,] [47]
Urachus, the, [317,] [341]
Urinary system, evolution of the, [335–42]
Urogenital ducts, [335]
Uterus masculinus, the, [344,] [350]
Utriculus, the, [289]
V
Vasa deferentia, [335]
Vascular layer, the, [16,] [168]
— system, evolution of the, [321–25]
— — structure of the, [318]
Vegetative layer, the, [16]
Veins, evolution of the, [323–24]
Ventral pedicle, the, [166]
Ventricles of the heart, [325]
Vermalia, [220,] [223]
Vermiform appendage, the, [32,] [310,] [317]
Vertebræ, [142,] [294]
Vertebræa, [105]
Vertebral arch, the, [148,] [295]
— column, the, [144]
— — evolution of the, [296]
— — structure of the, [294]
Vertebrates, character of the, [104–10]
— descent of the, [219–20]
Vertebration, [142]
Vesico-umbilical ligament, the, [341]
Vesicula prostatica, the, [344,] [350]
Villi of the chorion, [165]
Virchow, R., [35]
— on the ape-man, [303]
— on the evolution of man, [264]
Virgin-birth, [9,] [13]
Vitalism, [6]
Vitelline duct, the, [138]
Volvocina, [213]
W
Wallace, A. R., [29]
Water, organic importance of, [200]
Water vessels, [336]
Weismann’s theories, [349]
Wolff, C. F., [13]
Wolffian bodies, [339]
Wolffian duct, the, [341]
Womb, evolution of the, [342–43]
Y
Yelk, the, [43,] [45,] [67]
Yelk-sac, the, [117,] [134]
Z
Zona pellucida, the, [44]
Zonoplacenta, [255]
Zoomonera, [209]
Zoophytes, [20,] [64,] [104]