The Evolution of Man — Volume 2

by Ernst Haeckel

CHAPTER 2.28. 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 coeloma. 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.

(FIGURE 2.358. 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.)

FIGURE 2.359. 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 (haemocytes) 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 anaemia (or chlorosis).

(FIGURE 2.360. 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 coeloma (secondary body-cavity). So skin-fibre layer, Sp gut-fibre layer, v blood-vessels in latter, ao primitive aortas, containing red blood-cells.)

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 (Figure 2.358) and some groups of Annelids. When we examine the colourless blood of a cray-fish or a snail (Figure 2.358) under a high power of the microscope, we find in each drop numbers of mobile leucocytes, which behave just like independent Amoebae (Figure 1.17). 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 Amoebae, 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.

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 (haemoglobin) 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 (Figure 2.358). The mammals are distinguished from the other Vertebrates by the circular form of their biconcave red cells and by the absence of a nucleus (Figure 1.1); only a few genera still have the elliptic form inherited from the reptiles (Figure 1.2). In the embryos of the mammals the red cells have a nucleus and the power of increasing by cleavage (Figure 1.10).

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 (Figure 2.360 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 (Figure 2.361 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 Figure 1.152. 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.

(FIGURE 2.361. 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 apostrophe a deeper nucleus, in the act of cleavage, k asterisk 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.

(FIGURE 2.362. Vascular system of an Annelid (Saenuris), 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 (Figure 2.244). 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 haemoglobin, 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 (Figure 2.362). 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, Figure 2.245). 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.

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 (Figure 2.221). 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 (Chapter 2.16). 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).

(FIGURE 2.363. 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.)

FIGURE 2.364. The five arterial arches of the Craniotes (1 to 5) in their original disposition, a arterial cone or bulb, a double apostrophe aorta-trunk, c carotid artery (foremost continuation of the roots of the aorta). (From Rathke.)

FIGURE 2.365. The five arterial arches of the birds; the lighter parts of the structure disappear; only the shaded parts remain. Letters as in Figure 2.364. s subclavian arteries, p pulmonary artery, p apostrophe branches of same, c apostrophe outer carotid, c double apostrophe inner carotid. (From Rathke.)

FIGURE 2.366. The five arterial arches of mammals; letters as in Figure 2.365. v vertebral artery, b Botall's duct (open in the embryo, closed afterwards). (From Rathke.))

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 (Figure 2.359).

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. Figure 2.210). 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, Figures 1.98 to 1.102).

(FIGURES 2.367 TO 2.370. 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 Figure 2.367 only three, in Figure 2.368 all five, of the aortic arches are given (the dotted ones only are developed). In Figure 2.369 the first two pairs have disappeared again. In Figure 2.370 the permanent trunks of the artery are shown; the dotted parts disappear, s subclavian artery, v vertebral, ax axillary, c carotid (c apostrophe outer, c double apostrophe inner carotid), p pulmonary.)

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 amoeboid cells (leucocytes, Figure 1.12) 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, Figure 2.358). 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.

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 (Figure 2.363 abr). On each side 5 to 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 (Figure 2.364) the chief parts of the arterial system develop in all the higher Vertebrates.

(FIGURE 2.371. 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.)

FIGURE 2.372. Heart of the same embryo (Figure 2.371), from the front. v vitelline veins, a auricle, ca auricular canal, l left ventricle, r right ventricle, ta arterial bulb. (From Bischoff.))

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 (Figure 2.365 p). These force a part of the mixed blood into the lungs, the other part of it going through the aorta into the body.

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 (Figure 2.262), 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 (Figure 2.365). In the mammals this has been developed from the LEFT half of the same fourth arch (Figure 2.366).

(FIGURE 2.373. 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.)

FIGURE 2.374. 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)

FIGURE 2.375. Heart of a human embryo, four weeks old; 1. front view, 2. back view, 3. opened, and upper half of the atrium removed. a apostrophe left auricle, a double apostrophe right auricle, v apostrophe left ventricle, v double apostrophe right ventricle, ao arterial bulb, c superior vena cava (cd right, cs left), s rudiment of the interventricular wall. (From Kolliker.)

FIGURE 2.376. 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.)

FIGURE 2.377. Heart of a human embryo, eight weeks old, back view. a apostrophe left auricle, a double apostrophe right auricle, v apostrophe left ventricle, v double apostrophe right ventricle, cd apostrophe right superior vena cava, ci inferior vena cava. (From Kolliker.))

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 (Figures 2.367 to 2.370). 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, Figure 2.367). 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 Figures 2.364 and 2.368) the pulmonary arteries.

The human heart also develops in just the same way as that of the other mammals (Figure 2.378). We have already seen the first rudiments of its embryology, which in the main corresponds to its phylogeny (Figures 1.201 and 1.202). 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.

(FIGURE 2.378. 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 (Figures 2.371 and 2.372). 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 (Figure 2.371 b); and the constriction between the atrium and ventricle is called the auricular canal (Figure 2.372 ca). The heart of the human embryo is now a complete fish-heart.

(FIGURE 2.379. Transverse section of the back of the head of a chick-embryo, forty hours old. (From Kolliker.) m medulla oblongata, ph pharyngeal cavity (head-gut), h horny plate, h apostrophe thicker part of it, from which the auscultory pits afterwards develop, hp skin-fibre plate, hh cervical cavity (head-coelom or cardiocoel), hzp cardiac plate (the outermost mesodermic wall of the heart), connected by the ventral mesocardium (uhg) with the gut-fibre layer or visceral coelom-layer (dfp apostrophe), 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.)

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. Figures 2.373 to 2.378). 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, Figures 2.375 c and 2.377 c); the left auricle receives the pulmonary veins. In the same way a superficial interventricular furrow is soon seen in the ventricle (Figure 2.376 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 (Figure 2.376 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.

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, Figure 2.379 uhg). The mesocardium divides two lateral cavities, Remak's "neck-cavities" (Figure 2.379 hh). These cavities afterwards join and form the simple pericardial cavity, and are therefore called by Kolliker the "primitive pericardial cavities."

(FIGURE 2.380. Frontal section of a human embryo, one-twelfth of an inch long in the neck, magnified forty times; "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 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 coelom-pouches which comes next to Van Wijhe's "visceral cavities" below. Each of the cavities still communicates freely behind with the two coelom-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-coeloma," and by Hertwig the "pericardial breast-cavity." As it now encloses the heart, it may also be called cardiocoel.

The cardiocoel, or head-coelom, 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. Figure 1.180 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 (Figure 2.380 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-coelom, 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.

(FIGURE 2.381. 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 coelom, hk top of heart, ks head-sheath, amniotic fold, h horny plate. (From Remak.)

(FIGURE 2.382. 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.))

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-coelom, originating from the pleuroducts; the pericardium in which the heart afterwards lies is also double originally, being formed from ventral halves of the head-coelom, 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.

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 (Figure 1.134 h). So also the two coelom-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 cardiocoel disappears, the two cervical cavities freely communicate (Figure 2.381), 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 (Figure 2.381) 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 (Figures 2.379 s and 2.382 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 1.15), are late acquisitions and entirely cenogenetic. (Cf. Chapter 1.15 and Figures 1.198 to 1.202.)

In the Selachii also we find a longitudinal row of segmental canals on each side, which open outwards into the primitive renal ducts (nephrotomes, Chapter 1.14). 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 (Figures 2.386 and 2.387). 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 coelous 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 coelom-pouches arises between the limiting layers (Figure 2.385). 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 (Figure 2.387). 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.

(FIGURE 2.395. Primitive kidneys and germinal glands of a human embryo, three inches in length (beginning of the sixth week), magnified fifteen times. 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 (Figures 1.183 m, 1.184 m, 2.388 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.

(FIGURES 2.396 TO 2.398. Urinary and sexual organs of ox-embryos. Figure 2.396, female embryo one and a half inches long; Figure 2.397, male embryo, one and a half inches long. Figure 2.398 female embryo two and a half inches long. w primitive kidney, wg Wolffian duct, m Mullerian duct, m apostrophe upper end of same (opened at t), i lower and thicker part of same (rudiment of uterus), g genital cord, h testicle, (h apostrophe, lower and h double apostrophe, upper testicular ligament), o ovary, o apostrophe 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 Kolliker.))

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 (Figure 1.147 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.

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 Mullerian duct, after its discoverer, Johannes Muller, while the former is called the Wolffian duct. The origin of the Mullerian 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 Mullerian ducts." The latter (Figure 2.387 m) lies just on the inner side of the former (Figure 2.387 w). Both open behind into the cloaca.

However uncertain the origin of the nephroduct and its two products, the Mullerian 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 Mullerian 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 Mullerian ducts, which I have called "Rathke's canals" (Figure 2.394 c). In the female sex, in which the two Mullerian ducts form the oviducts, there are relics of the Wolffian ducts, which are called "the ducts of Gaertner."

(FIGURE 2.399. Female sexual organs of a Monotreme (Ornithorhynchus, Figure 2.269). o ovaries, t oviducts, u womb, sug urogenital sinus; at u apostrophe is the outlet of the two wombs, and between them the bladder (vu). cl cloaca. (From Gegenbaur.)

FIGURES 2.400 AND 2.401. Original position of the sexual glands in the ventral cavity of the human embryo (three months old).

FIGURE 2.400 male (natural size). h testicles, gh conducting ligament of the testicles, wg spermaduct, h bladder, uh inferior vena cava, nn accessory kidneys, n kidneys.

FIGURE 2.401 female, slightly magnified. r round maternal ligament (underneath it the bladder, over it the ovaries). r apostrophe kidneys, s accessory kidneys, c caecum, o small reticle, om large reticle (stomach between the two), l spleen. (From Kolliker.))

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 (Figures 2.390 to 2.395). The first structure of the nephroduct and its differentiation into Mullerian and Wolffian ducts are just the same in both sexes in the Amphibia, as in the mammal embryos (Figures 2.392 and 2.396). In the female Amphibia the Mullerian duct develops on either side into a large oviduct (Figure 2.393 od), while the Wolffian duct acts permanently as ureter (u). In the male Amphibia the Mullerian duct only remains as a rudimentary organ without any functional significance, as Rathke's canal (Figure 2.394 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.

In the mammals these permanent amphibian features are only seen as brief phases of the earlier period of embryonic development (Figure 2.392). 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.

(FIGURE 2.402. Urogenital system of a human embryo of three inches in length, double natural size. 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 Mullerian 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 (Figure 2.399 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 Mullerian 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.

(FIGURES 2.403 TO 2.406. Origin of human ova in the female ovary.

FIGURE 2.403. 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 (Pfluger'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.)

FIGURE 2.404. 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).

FIGURES 2.405 AND 2.406. Two older Graafian follicles, in which fluid is beginning to accumulate inside the eccentrically thickened epithelial mass of the follicle-cells (Figure 2.405 with little, 2.406 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 Mullerian and Wolffian ducts at their lower ends. Here again they form a single genital cord (Figure 2.397 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 Mullerian 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).

(FIGURE 2.407. 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 (Figures 2.386 g and 2.392 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 (Figure 2.400 gh), and in the female as the "round maternal ligament" (Figure 2.401 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 (Chapter 2.22). Among the mammals this arrangement is permanent only in the Monotremes, which take their name from it (Figure 2.399 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, Figure 2.402 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 (Chapter 2.25); 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.

(FIGURE 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, magnified 250 times.) (Cf. Figures 1.1 and 1.14.)

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 proctodaeum, the ectodermic invagination of the rectum (Chapter 2.27); 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 papillae of the corium of the phallus, and have been evolved from ordinary tactile corpuscles of the corium by erotic adaptation (Chapter 2.25).

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 (nymphae). 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 (Chapter 1.3). Each follicle (Figure 2.407) 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 (Figure 2.387 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 (Figure 2.403 b). Some of the cells of these strings (or Pfluger'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 (Figure 2.404 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 (Figures 2.405 and 2.406). 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 (Pfluger'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.