ORGANIC CELLS: THE VISIBLE UNITS OF LIFE.
In the sanctuary of S. Vitale at Ravenna, in Italy, is a very interesting representation, in mosaic pictures, and over life-size, of the Emperor Justinian and his Empress Theodora, attended by a numerous suite of ladies and courtiers. The mosaics are small bits of glass, of varying pattern and color, cemented together so nicely as to form beautiful delineations of the Emperor and his attendants.
The bits of glass or mosaics that form the figures may very appropriately be called structural units.
The body of man, a bird, a lizard, an oak tree, and many other animals and plants, may usefully be compared to such mosaic figures; for, just as the mosaic figure has its structural unit, the little bit of glass or stone, called the mosaic; so the bodies of men, birds, and other creatures may be looked upon as infinitely complex figures formed of minute mosaics called cells.
The groups of minute mosaics or cells that make up the bodies of animals and plants differ profoundly from the mosaics that form the figures of the dead Emperor and his companions, inasmuch as each mosaic or cell of the animal or plant body is so small as to require the microscope to reveal it; also each mosaic of the living animal or plant is a living mosaic or cell, in that it can absorb food, digest it, assimilate it, grow, and multiply in numbers. Cells, then, are the morphological or structural units which compose the bodies of all living creatures.
All animals and plants begin life as single cells, which, in the vast majority of cases, are microscopic in size. Those that remain single cells, and dissociated throughout life, are called Unicellular Animals (Protozoa) and Plants (Protophyta), and are to be seen mostly by the microscope alone; but those which, by multiplication and growth, form large numbers of cells that remain associated together as in the body of a bird or lizard, are called Multicellular Animals (Metazoa) or Plants (Metaphyta).
A cell ([Fig. 1]) is a nucleated lump of protoplasm, or cytoplasm, and most often of microscopic size and more or less covered on its exterior by, and holding in its interior, various products and formations resulting from its activity, which are called metaplasm. Since the protoplasm of a cell, under the microscope, presents a superficial resemblance to a minute speck of that jelly-like substance (albumen) which forms the white of an egg, it is often called an albumenoid substance. But it is very misleading to use such an expression, for protoplasm is not a single chemical substance of great complexity; but it is rather composed of a large number of different chemical substances of great complexity. Many of these substances, it is true, are albumenoid in character. The same is true as to the chemical complexity of the nucleus, which is a physically and chemically differentiated part of the protoplasm.
The protoplasm contains certain globulins, and also albumins and peptones; it also contains large quantities of nucleo-albumins, with other substances. The nucleus not only contains these same substances, but also nuclein and nucleo-proteids. It is important to state that nuclein consists of an albumin and nucleic acid.
Fig. 1.—Diagram of a Cell, highly magnified.
The protoplasm, structurally, is made up of threads forming a complex, sponge-like substance, or reticulum, called spongioplasm; and in the meshes of the spongioplasm is a more or less fluid-like substance known as hyaloplasm: suspended in the hyaloplasm are various kinds of living bodies known as plastids, besides various products resulting from the activity of the protoplasm and which are designated metaplasm.
In many cells the protoplasm has formed on its periphery a layer of metaplasm which is frequently called a cell-wall. This cell-wall prevents amœboid movements of the protoplasm, and a cell possessing it is said to be encysted.
In many cells, especially vegetable ones, will also be observed clear spaces termed vacuoles. These vacuoles contain water with various chemical substances held in solution, which serve the purpose chiefly of food-reservoirs.
The nucleus also is formed of threads called nuclear or chromatin threads (chromosomes), the interstices of which are filled with hyaloplasm or achromatin. In the nucleus can also be observed the nucleolus.
The protoplasmic and nuclear threads show various structural modifications in different regions and under different physiological states of the cell.
As will be observed later on, the nuclear threads are of special interest to the student of heredity. They may in one phase of cell-activity look like one thread forming an inextricable network, while in other phases they may look like thick, short, distinct rods.
The centrosome ([Fig. 1]), with its enveloping attraction-sphere, constitutes another fundamentally important part of the cell. It is especially concerned with the phenomena of cell division and multiplication.
Just as the living body consists of an infinitely complex figure of living mosaics termed cells, so the cell itself consists of an infinitely complex figure of still smaller living mosaics called, by Spencer, Physiological Units. These units have been given different names by various writers, viz.: by Darwin, gemmæ (gemmules); by de Vries, pangennæ; by Hertwig, idioblasts; by Weismann, biophors, etc., etc.
Like the atom of the chemist and the molecule of the physicist, the physiological unit of the biologist is merely at present an intellectual conception, yet it is, at the same time, an intellectual necessity and plays a very important part as the theoretical component of many vital questions. Just as the cells are the visible units of life, so the physiological units are the invisible units.
The physiological activities of cells are those that pertain to their nutrition and reproduction.
The nutrition of cells includes all processes that are subservient to their life and well-being, such as irritability, contractility, absorption of food, its digestion and assimilation, secretion, etc.
In consequence of the wonderful nutritive activities of cells, we may well speak of them as marvelous magicians. Hertwig, following Haeckel, speaks of many cells as being builders. In the same spirit, we can say that multitudes of cells are expert chemists, artists, sculptors, mathematicians, and so on, in that they make all the myriad chemical products of organic nature, such as spices, pigments, sugars, starches, acids, perfumes, and numerous other substances; they paint in colors that rival the hues of the rainbow; they construct all of the beautiful forms in the animal and plant worlds; and they draw lines as straight and curves as graceful as the most expert mathematician.
One of the most important reproductive activities of a cell is mitosis (see below). Mitosis essentially consists of a series of processes by which each nuclear thread of the nucleus splits longitudinally into two equal parts, and then these equivalent parts separate from each other, so that from the one nucleus we get two smaller nuclei. Then each of these smaller nuclei appropriates its share of the enveloping protoplasm, finally splitting it into two parts. Thus from the larger cell (nucleated piece of protoplasm) we get two smaller cells (two smaller nucleated pieces of protoplasm). In technical language, we say that the larger cell is the mother cell, and the two smaller cells that it has divided into are the daughter cells. In consequence of the method of mitosis, the two daughter cells very frequently are exactly like the mother cell, except in size. But by the absorption of nutriment, and through digestion and assimilation, they grow and finally become exactly like the mother cell. This is the simplest illustration of heredity. The reproductive process may be repeated very many times, so that from one cell we may get millions of cells.[1]
It is necessary to assume that the nutritive and reproductive activities of cells are based upon and controlled by the nutritive and reproductive activities of the physiological units, inasmuch as these are the ultimate living units.
In the activities of a cell the nucleus and protoplasm are intimately correlated with one another.
The nucleus is looked upon by the majority of cytologists as the formative center of the cell in a chemical, and also, consequently, in a morphological, sense. Active exchanges of material take place between the nucleus and the protoplasm during the nutritive processes of the cell. Possibly this may be altogether a chemical process, or possibly it may be due, as Hertwig suggests, to the migrations of the physiological units as carriers and elaborators.
In these exchanges, and in the upbuilding chemical activities (anabolism) of the cell, the nucleic acid plays a leading part. Here the nucleic acid in the physiological units of the nuclear threads, combines with albumins from the protoplasm, forming nuclein. Much of this nuclein, undergoing further elaboration, is passed into the protoplasm as one of its finished products (metaplasm). The more purely nutritive the activity of a cell, the more nuclein its nuclear threads contain; on the other hand, when the cell is in the phase of reproductive activity, the nucleus contains little nuclein, and is almost entirely composed of pure nucleic acid.
Fig. 2.—Stylonychia: c, an entire animal, showing planes of section; the middle piece of c contains two nuclei and can regenerate a perfect animal; a, and b, contain no nuclei,—they live and swim about for a while and then die.
That the nucleus is the formative center of the cell is indicated by the following, among many facts: If a unicellular animal, such as Stylonychia ([Fig. 2]), for instance, be broken up into several fragments, it will be observed that some of the fragments are nucleated and others non-nucleated. The nucleated fragments have the power of quickly healing the wounds on them, regenerating the missing parts, and thus restoring the mutilated fragments to perfect individuals. These nucleated fragments have the power to perform all the activities of the perfect animal. The non-nucleated portions, on the other hand, cannot undergo regeneration. They cannot digest food, or grow or secrete substances as the nucleated fragments can. They can simply live for awhile, responding to stimuli and moving about. They finally perish.
Having mentioned in a general way some of the wonderful powers of cells, it will be well now to describe briefly a few of the unicellular plants and animals that can be so easily obtained and studied in warm weather, and which may thus serve as illustrations of the powers of nucleated pieces of protoplasm or cells. Many unicellular plants and animals can be obtained in summer from the superficial ooze on the bottom of slow-running streams and also on the under surfaces of the leaves of water plants, a study of which will be of the greatest value and interest.
Amœba proteus ([Fig. 3]). This little unicellular animal, which belongs to the Rhizopod type, is very common in ponds and streams in warm weather. In the resting state it is spherical in form, but when active its form is as changeable as the fabled Proteus, hence its name, Amœba proteus. This little creature is a naked piece of protoplasm, with its outer layer differentiated into a firmer and pellucid part called the ectoplasm ([Fig. 3], ec); its interior, the endoplasm (en), is quite granular and much more fluid, the granular particles moving quite freely upon one another when the animal changes its shape. The superficial portion of the endoplasm is firmer than its more central parts, and graduates insensibly into the more consistent ectoplasm.
Fig. 3.—Amœba proteus: n, nucleus; cv, contractile vesicle; ec, ectoplasm; en, endoplasm; p, pseudopodia.
In the periphery of the granular endoplasm, and adherent to the inner surface of the ectoplasm, is a clearly defined nucleus (n). When most distinctly seen, it presents the appearance of a clear vesicle surrounding a solid and more or less spherical nucleolus. A contractile vacuole (cv) is also uniformly present, located in the endoplasm. The creature has the power of putting out projections (p) from the surface called false feet (pseudopodia). Sometimes the protrusion consists of ectoplasm alone, but more commonly endoplasm extends into it, when a current of granules will be observed moving from the more central portions of the Amœba into its protrusion, whilst from some other protrusion that is being withdrawn a similar current may set towards the center of the body, and thus the animal moves, in a creeping manner, from place to place. While moving about in this way the little animal comes across other one-celled creatures, such as Desmids and Diatoms, seizing them and forcing them through its ectoplasm into the endoplasm, where the nutritious parts are digested and assimilated. After the animal has taken its prey through its ectoplasm, no break in the continuity of the ectoplasm remains, but the parts immediately come together in a perfect manner. After it has abstracted all the nutriment from its prey, the Amœba casts away from it the parts that are indigestible.
Fig. 4.—Rotalia Freyeri: a many-shelled Foraminifer, or a colony of many single-shelled Foraminifera, with pseudopodia extended.
Foraminifera. These are little protoplasmic unicellular animals that have the power of secreting for themselves more or less complex envelopes composed of limestone. They may be single, as in Lagena, or composed of a number of individuals with the shells cemented together as in Globigerina or Rotalia ([Fig. 4]). They have played a part of vast importance in the geological development of the world. Their myriads of shells remaining at the bottom of seas millions of years after the little protoplasmic bodies have perished, they have been consolidated into vast expanses of limestone rocks, and finally uplifted into such formations as the huge chalk cliffs of England.
Osteoblasts and Osteoclasts. These cells are naked pieces of protoplasm, the latter much the larger and having many nuclei. They are concerned in some of the most interesting phenomena of many growing animals. Just as the Foraminifera have the power of forming complex aggregations of limestone shells, so the Osteoblasts have the power to construct the bones of animals. And when a bone is broken as the result of accident, these little cells do the mending. While the Osteoblasts are bone-formers, the Osteoclasts are bone-destroyers. It is very curious that little specks of living jelly, like these Osteoclasts, should have the power of destroying hard tissue like bone, but such is the fact. These Osteoclasts can, by their wonderful chemical processes, liquefy and absorb, and by these means destroy, ivory pegs that are driven into living bone. They are the agents by which the roots of children’s milk teeth are destroyed, so that the crowns of the teeth are shed and the way paved for the appearance of the permanent teeth. The wonderful activity of these little Osteoblasts and Osteoclasts is well exemplified in the growth and shedding of the antlers of deer. While these antlers are growing in the spring, they are covered with a delicate skin, technically called “velvet.” This velvet is very sensitive and quite warm from the nutrient blood circulating through it. In it are hundreds and thousands of busy, living Osteoblasts that work together under some mysterious, directing or coördinating agency, to build up the splendid beams, tynes and snags that constitute the antlers, which in many deer of the Rocky Mountains reach such a size that a man may walk under the archway made by setting the shed antlers up on their points. No hive of bees is busier or more replete with active life than the antler of a stag as it grows beneath the warm, soft velvet, through the agency of the Osteoblasts.
The building of the antlers by these little agents continues through the spring and summer. In the autumn the Osteoblasts cease their activity and die; the delicate, sensitive velvet dries and peels off, leaving the dead, hard, bony substance exposed, and they now become weapons adapted for fighting. This is the season when the stags challenge one another to single combat, the hinds standing timidly by to be taken by the victor as his mates. When the loves and battles of the autumn are over and the mating is completed, the antlers no longer serve a useful purpose, and they are shed. The shedding is accomplished through the agency of the bone-destroyers, the little jelly-like cells called Osteoclasts.
Bacteria are exceedingly minute specks of naked protoplasm. They are unicellular plants. Some of them are harmless to mankind; some are very useful to him, and others are his deadly enemies. Many of them are concerned in the production of the infectious diseases. They do so by elaborating various chemical products that are virulent poisons, hence these products are called toxines; when taken up by the blood, they are carried to various parts of the body. In this manner they cause the particular symptoms that are characteristic of a special infectious disease. Why is it that some persons, on exposure to an infectious disease, contract the malady while others similarly exposed do not? In other words, what gives immunity to disease? The explanation is probably as follows: Just as the invading bacteria have the power of secreting toxines, so the cells of the body, normally, have the power of elaborating chemical products that are antidotes to the toxines, and are appropriately called antitoxines. Infectious diseases and immunity from them, are the result of a contest between the invading bacteria and the protecting cells of the body. If the bacteria secrete toxines in greater quantities than can be neutralized by the cells of the body, we have disease; if the reverse occur, we have immunity.
The white blood-corpuscles (cells) also take part in this warfare. They have the power of traveling, in virtue of their amœboid movements, from the blood to the part invaded by the bacteria. Here a contest takes place between them, the corpuscle takes the bacteria into its interior, and either kills them or is itself killed. The result of this contest helps to produce either immunity or infection.
Tetanus bacillus is a cell shaped like a slender rod. It has the power of secreting a poison which, when introduced into the body, produces convulsions and other symptoms of lockjaw. These much resemble those induced by strychnine poisoning.
Bacillus diphtheriæ is an exceedingly small unicellular plant, and has the power of producing a poison called toxalbumin, which is analogous to the poison of certain venomous serpents. It is the speck of protoplasm through whose activity diphtheria is caused.
Many useful bacteria have the power of so acting on dead organic bodies as to decompose them, the three most conspicuous end-products of this decomposition being water, carbonic oxide and ammonia. When the dead bodies are decomposed in the soil there are other bacteria, in addition, that have the power of further acting on the ammonia, causing its oxidation and producing nitrous and nitric acids and their salts. The unicellular plants that bring about these changes are the nitrifying bacteria. Conspicuous illustrations of the functional activity of these little naked pieces of protoplasm are seen in the immense saltpeter beds of Peru and Chili, where, from the enormous fecal accumulations of sea-fowls, the immense quantities of nitrates are produced that supply the commercial world.
Fig. 5.—Difflugia Pyriformis.
Arcella, of which there are many species, is a unicellular animal whose protoplasmic body has secreted from its surface an enclosing “test” that is composed of a horny membrane, resembling very much in constitution the chitin which gives firmness to the integuments of insects. This creature is commonly discoidal in shape, with one face arched and the other flat, an aperture being situated in the center of the flat side through which the creature may thrust its pseudopodia or withdraw them. The surface of the testaceous covering is often marked with a regular but minute and attractive pattern. In Difflugia ([Fig. 5]), the test is somewhat pitcher-shaped, and is mostly made up (by the constructive activity of the protoplasm) of exceedingly small particles of shell and gravel cemented together. Many testaceous amœbans form tests of singular beauty and remarkable regularity. In some of the animals the minute plates of which the tests are formed have been picked up from the surface over which the animals crawl, and are cemented into various charming patterns; and in other cases they are formed by secretion from their own bodies. In Quadrula symmetrica the protoplasmic body has constructed a pear-shaped testaceous covering, of complete transparence-like glass, composed of a great number of square plates touching each other by their edges. The protoplasmic body of the animal does not entirely fill the test, the intervening space being occupied by a clear liquid and traversed by bands of protoplasm. A clear, large spherical nucleus is seen in the part farthest from the pseudopodia. It contains a dark and well-defined nucleolus. In front of the nucleus two contractile vesicles are to be observed. The pseudopodia in these creatures, it must be remembered, are not appendages, but lobate protrusions of the protoplasmic body, are few in number, rounded, short and broad.
Diatoms are unicellular plants, isolated or aggregated together, that have the power of constructing flint coverings, often of great complexity and charming pattern. The tracings on many of these flint coverings are so constant and small, that they are frequently employed for the purpose of testing the power of modern compound microscopes. In various parts of the world vast deposits of Diatoms have been discovered. The most remarkable of these for extent, as well as for the beauty and number of the species contained in it, is that on which the city of Richmond, in Virginia, is built, which is over thirty feet deep and extends for many miles.
Fig. 6.—Noctiluca miliaris. A, dorsal view; B, side view; n, nucleus; f, flagellum; a, entrance to atrium; b, atrium; o, œsophagus; r, superficial ridge.
Noctiluca miliaris ([Fig. 6]) is a very large unicellular, flagellate animal. It is spheroidal in form, and has an average diameter of not quite one-half a millimeter. It is just large enough to be observed by the unaided eye when the water in which the animal may be swimming is contained in a glass jar held up to the light. It has a tail-like appendage (flagellum) by which the animal moves about. Along one side of the cell is a meridional groove resembling that of a peach, and leading into a deep depression of the surface termed the atrium ([Fig. 6], B, b). It is from the shallow commencement of this depression that the flagellum ([Fig. 6], f) originates. At the base of the flagellum the depression sinks down to the mouth (o). A slightly elevated ridge (r) extends along the opposite meridian and commences with a bifurcation at that end of the atrium farthest from the flagellum. The mouth opens into a short œsophagus, which leads down directly to the central protoplasmic mass. The central protoplasmic mass sends off branching prolongations of its substance in all directions, the ramifications of which freely inosculate. The farther these ramifications extend out to the periphery, the thinner they become, until finally a protoplasmic network of extreme tenuity is formed immediately under the enveloping membrane of the cell. In addition to these ramifying prolongations, the central protoplasmic mass sends off a thin, broad, irregular extension to the superficial ridge and coalesces with it. Near the central protoplasmic mass is seen the nucleus (n).
The flagellum is a flattened, whip-like filament, having a striated appearance, and gradually tapers from the base to its extremity. It slowly bends over five or six times a minute to the mouth, and then, more slowly still, bends away again. It is through the movements of the flagellum that particles of food are driven into the mouth and down the œsophagus into the central protoplasmic mass. In this mass and its extensions the food is digested and assimilated.
This little one-celled animal has the power, through its special chemical activities, of manufacturing and emitting light. It is through the agency of myriads of these little creatures that the diffused luminosity of some seas is produced and can be observed at night. The Noctiluca is very transparent, and for this reason it is a particularly favorable subject for the study of its luminosity or phosphorescence. They can be obtained by the tow-net in unlimited quantities from the sea and transferred into a jar of sea water. Here they soon rise to the surface, forming a thick layer. If the jar be placed in the dark and agitated in the slightest degree, there is an instantaneous display of light, which is of a beautiful greenish tint. The light emitted by the Noctiluca is so vivid that it can even be observed in ordinary lamp-light. This phosphorescence is only of an instant’s duration, and a short rest is necessary for its renewal. The special locality for the formation of the phosphorescence is in the very fine protoplasmic network, which lines the external structureless membrane or cell wall. These wonderful little Noctilucæ may well be figuratively called the fire-flies of the ocean.
Fig. 7.—Gromia oviformis with protoplasmic threads (pseudopodia) extended and forming an elaborate network in which a captured unicellular organism is seen; d, diatom captured; p, protoplasm containing captured diatoms; s, shell.
Gromia oviformis ([Fig. 7]) is often found in fresh water adhering to confervæ and other plants of running streams. The protoplasmic body of this animal is enveloped in a chitinous covering that is egg-shaped and of a brownish-yellow color. It is about two millimeters in diameter. When the animal is quiet, no one would suspect its real nature, so much does it look like the seed of an aquatic plant. The testaceous envelope has a single round orifice at its more pointed end. The animal, when in an active state, pushes out the protoplasmic substance, which speedily gives off ramifying extensions, and these by further ramification and inosculation form a complicated network. The protoplasm of the animal also extends itself in such a way as to form a continuous layer on the external surface of the test. From this layer numerous protoplasmic threads may extend out, forming more or less complicated networks. By the alternate contraction and extension of its protoplasmic threads and networks, minute one-celled plants and animals are entrapped like flies in a spider’s web (d). When caught they are carried, by retraction of the protoplasmic thread-like pseudopodia, into the endoplasm in the test. Here the nutritious parts of the entrapped creatures are abstracted and assimilated. In transparent species, the indigestible parts, such as the silicious valves of diatoms, may be distinguished in the midst of the endoplasm, from which they are ultimately extruded.
When gromia oviformis reproduces by mitosis it gives off a bud (small cell), which finally separates from the parent form and constitutes a distinct individual. This process may be repeated many times, so that a great number of separate individuals may be formed, all of which lead detached and independent lives.
Most of the protozoa, which are produced by fission (cell division by mitosis), separate entirely from each other, as in Gromia; but in many of these unicellular animals, the new creatures produced by fission do not separate from one another, but remain more or less closely connected, and thus form colonies of Protozoans. These colonies are of the greatest interest, for they represent a lower stage of the cell colonies of the Metazoa (multicellular animals). They reproduce, in many cases, in a way which is strongly suggestive of reproduction in the Metazoa.
Microgromia socialis is a little unicellular animal, having a thin, nearly globular, calcareous shell that it secretes upon its surface. It multiplies by fission, and forms a number of distinct individuals which have the curious habit of fusing their pseudopodia and uniting into a more or less closely associated colony. The individuals sometimes remain at a distance from one another, but sometimes associate themselves together into a compact colony. These individuals are all alike, performing the same functions. There is no division of labor among the units, but they live practically an independent life. If the individual animals were detached from one another, they would live and build new colonies.
Codosiga umbellata is another unicellular animal—a flagellate Protozoan. It has a collar-like extension of its ectoplasm from the anterior extremity of its body, forming a sort of funnel from the bottom of which the thread-like structure (flagellum) arises. The vibrations of this flagellum cause a current of the surrounding fluid to set into the funnel so that particles of food reach the soft protoplasmic substance which serves as a mouth. The nucleus is seen near the base of the collar. Near the posterior extremity of the body two contractile vesicles are to be observed. This posterior extremity of the animal has a cylindrical extension of its ectoplasm by which it attaches itself to an object. This protozoan multiplies by longitudinal fission. In some species the animals separate completely from one another and lead entirely independent lives. But in Codosiga umbellata the fission does not extend through the cylindrical extension, so that a group of animals are associated in a colony.
Rotalia or Globigerina. The shelled amœba (Lagena) gives off a bud (unseparated cell) which grows to the full size, secretes its calcareous shell and remains connected with the parent form. This process may be repeated a number of times until a colony of shelled amœbæ, of varying pattern in different species, may be formed and permanently associated together ([Fig. 4]). The individual amœbæ are all alike and perform the same functions. There is no division of labor, no specialization, among them. If the individual animals could be separated from one another they would live and build new colonies.
Pandorina morum forms a small colony of sixteen cells (solid sphere) of mulberry-like shape and enclosed in a common gelatinous envelope. Each cell in the mulberry mass bears two flagella on its peripheral end. These project out beyond the surface of the gelatinous envelope, and are agents for locomotion of the colony. The cells in the colony are all alike. There is no division of labor among them. They all act alike. The cells (flagellate protozoans) of the colony may reproduce in two ways. Each animal in the colony may subdivide into sixteen smaller units, each of which by growth and multiplication may form a new mulberry mass, a new colony, each unit of which acquires two flagella. Or two of the small units may amalgamate (conjugate), and then develop (by fission) into a new colony. The conjugating units are nearly of the same size and look very much alike.
Volvox globator is a spheroidal shaped colony (hollow sphere) of unicellular flagellate animals, about one-half a millimeter in size. It was formerly supposed to be a fresh-water Alga. It is now known to be a colony of Protozoans. All the animals in this colony are not alike. There is a division of labor among the cells, for some are merely vegetative, serving purposes of nutrition, and having no reproductive powers; while other members of the colony are purely reproductive animals. Furthermore, there is quite a marked specialization of the reproductive cells. Those reproductive cells that may be spoken of as the female cells are large and non-motile encysted cells. The male cells are small and actively motile, in that they have two flagella developed on them. A small flagellate male cell penetrates the large encysted female cell, and as the result of this conjugation, fission takes place repeatedly, and a new colony of flagellate protozoans (volvox) is formed. Volvox approaches very suggestively towards the type of animals known as Metazoan.
In order to comprehend, in some measure, the transition from Colonial Protozoa to Metazoa, it will be well for the reader to study a typical sponge. For a long time the Porifera (Sponges) were looked upon as compound Protozoa (colonial Protozoa), but while they are nearer the Protozoa than any of the other types of Metazoa, their position in the animal series is unquestionably among the Metazoa. The Sponge, like the rest of the Metazoa, develops from a fertilized egg by a process of cell multiplication, differentiation, gastrulation, etc.
CELL REPRODUCTION BY MITOSIS.
The multiplication of cells plays a part of such fundamental importance in Evolution, and therefore in Embryology and studies in Heredity, that it is necessary to study the subject somewhat in detail. It is a wonderful process, and is worthy of very careful attention.
Fig. 8.—Diagram illustrating Mitosis. A, the cell commencing activity; B, C, D, phases in the formation of the spindle and the chromatin loops or V’s, also showing that the mother V’s have split into daughter V’s; D, the chromatin loops forming the equatorial plate, chr; E, F, G, separation of the daughter loops (daughter chromosomes) and their passage towards the poles of the spindle, thus forming daughter nuclei; H, I, division of the protoplasm so as to form two daughter cells; at, attraction sphere enclosing a centrosome; n m, nuclear membrane; chr., chromatin threads; p, protoplasm; c w, cell wall; sp, spindle.
The process by which one cell (a mother cell) divides into two cells (daughter cells) is called mitosis, and is inaugurated by the centrosome ([Fig. 8], A, at). The centrosome divides into two centrosomes, which at first remain close together ([Fig. 8], B, sp), and then gradually separate from one another. Each centrosome becomes the center of a system of fine achromatin fibers arranged round it in a radiating manner and forming what is called the attraction sphere; also, at the same time, a spindle-shaped bundle of achromatin fibers, called the spindle ([Fig. 8], B, sp), extends between the centrosomes. In the meantime, important changes have been taking place in the chromosomes (hereditary threads) of the nucleus. The chromosomes, which at first are arranged in an apparently inextricable tangle or network, frequently assume U-shaped or V-shaped forms ([Fig. 8], C, chr), and the nuclear membrane disappears. Sooner or later each chromosome splits longitudinally into two daughter chromosomes, with which the achromatin fibers of the spindle become connected ([Fig. 8], D). In this phase of mitosis the split V-shaped chromosomes form a single group called the equatorial plate (chr), and extend across the axis of the spindle. It is to be observed from the diagrams in the figure, that one of the centrosomes has traveled to the opposite pole of the nucleus, thus causing the achromatin fibers of the spindle to extend across the original site of the nucleus. The equatorial plate of split V-shaped mother chromosomes (hereditary threads) thus divides the fibers of the spindle into two parts, one half extending from one centrosome to one group of daughter chromosomes, while the remaining half extend from the other centrosome to the other group of daughter chromosomes. Soon the achromatin fibers of the spindle contract, and in this way separate the two groups of daughter chromosomes, so that one group is drawn towards one centrosome, and the other group to the other centrosome ([Fig. 8], E, F, G, H). After the two groups of daughter chromosomes have been drawn to their respective centrosomes, each group assumes the tangle or network phase like the nucleus of the mother cell, and an investing nuclear membrane reappears for each ([Fig. 8], I). Thus from the mother nucleus of the mother cell we get two daughter nuclei (I). In a further phase of the mitotic process, a furrow appears on the surface of the protoplasm and surrounds it in the form of a ring. This furrow is in a plane at right angles to the long axis of the spindle, and gradually deepens until the protoplasm is divided into two parts, each segment of protoplasm containing its own nucleus and centrosome; in short, the mother cell has divided into two daughter cells (I).
It will thus be observed that the centrosomes and their achromatin fibers are a beautiful mechanism by which the heredity threads (chromosomes) are exactly divided into two equivalent halves.
There are some cells (Amœba proteus, for instance,) which divide in a much simpler manner than by mitosis; in these there is no complicated rearrangement of the chromosomes and no disappearance of the nuclear membrane, the nucleus simply becoming separated into two parts (Amitosis).
The Human Ovum. The human ovum is a typical cell about one-fifth of a millimeter in size and spherical in shape. It is a nucleated piece of protoplasm possessing an enveloping cell-wall (metaplasm). The protoplasm contains nutrient material or yolk (metaplasm). The maturation of the ovum essentially consists in throwing out half of its chromosomes. In doing this the nucleus (mother) approaches the surface of the protoplasm ([Fig. 9], A), and divides, by mitosis, into two daughter nuclei; then the unripe ovum divides into two cells, but of very unequal size. This process is repeated a second time. Thus two small cells are formed which are known as the polar bodies ([Fig. 9], A, B, pol. b). The large cell remaining after the formation of the two polar bodies is the mature ovum (B). Its nucleus, which recedes towards the center of the protoplasm, is called the female pronucleus ([Fig. 9], B, f. pr.). The pronuclei of mature ova differ from the nuclei of all the other cells of the body in that they only contain half as many chromosomes (hereditary threads).
The ripe spermatozoid (a flagellate sexual cell of the male) corresponds to the ovum of the female. It also contains a pronucleus having only half the number of chromosomes that the other cells of the adult body possess.
Fertilization. The male sexual cell (spermatozoid) is vastly smaller than the female sexual cell (ovum). Having a flagellum ([Fig. 9], B, s), it moves about, like a tadpole in water, and seeks the ovum. When it comes in contact with the ovum it penetrates into its interior (usually only one doing so), as indicated at ([Fig. 9], B, s). The tail or flagellum of the spermatozoid fuses with the protoplasm of the ovum, and disappears from view. Its pronucleus (C, m. pr.), accompanied by its centrosome (C, m. c.), approaches the female pronucleus (f. pr.) of the ovum ([Fig. 9], C, D). Finally the male and female pronuclei coalesce to form a single nucleus ([Fig. 9], E, n. os.). The centrosome of the ovum persists for awhile and then disappears; that of the spermatozoid remains in the ovum, and is the agency by which cell multiplication, through mitosis, takes place.
Fig. 9.—Diagram illustrating the maturation and fertilization of the human ovum. A, one polar body is formed and a second is in process of formation; B, both polar bodies are formed and a spermatozoid is penetrating the ovum; C and D represent the approach of the male pronucleus towards the female pronucleus; E indicates the amalgamation of the two pronuclei to form the nucleus of the oösperm (segmentation nucleus); pol. b, polar bodies; pol. c, centrosome of the polar body; chr. p, chromatin of the polar body; f pr, female pronucleus; p, protoplasm; p p, peripheral protoplasm (but not cell wall); f c, female centrosome; m c, male centrosome; m pr, male pronucleus; n, os, nucleus of the oösperm (first stage of a human being).
The cell resulting from the coalescence of the male pronucleus with the pronucleus of the ovum, and which is only one fifth of a millimeter in size, is the first stage in the existence of a human being. Man thus starts his career as a Protozoan-like creature,—as a unicellular animal. The fertilized ovum is called the sperm-egg (oösperm), and contains now the normal number of hereditary threads (chromosomes); for those of the male have been added to those of the female.
Segmentation of the Oösperm. The fertilization of the ovum imparts to it a wonderful stimulus, so that the oösperm divides, by mitosis, into two cells, these two into four, the four into eight, the eight into sixteen, these into thirty-two, and so on repeatedly, until a large number of comparatively small cells are formed ([Fig. 10]). This mass of cells is spherical in shape, and the little round cells towards the surface project in such a way as to give to the mass an appearance somewhat similar to the fruit of the mulberry, whence it is termed the mulberry body or morula ([Fig. 10], 4). In the morula stage of his existence man resembles the solid colony of protozoans represented by Pandorina. The cells of the morula next become arranged regularly in a single layer at the circumference, by which the embryo assumes the form of a hollow sphere, and is known as the blastula ([Fig. 10], 5). This phase of man’s existence is quite suggestive of Volvox.
Soon one side of the blastula is invaginated or pushed in, as one would push in one side of a hollow india-rubber ball. The result of this invagination (called gastrulation technically) is the formation of a sort of cup. This is the gastrula ([Fig. 10], 6) phase of man’s existence. It is a higher and fundamentally different phase of existence than either the morula or the blastula. It corresponds, not to a Protozoan, but to the higher Metazoan. It possesses the fundamental anatomical qualities of a low Cœlenterate (Polyp).
The mode of gastrulation is different in man from that just described, and varies in different animals, but the essential point common to all is the formation of a double cellular-membrane; the outer membrane being called epiblast or ectoderm, and the inner one hypoblast or endoderm, the enclosed cavity being the primitive digestive cavity. These two layers are the primary germinal layers. A third layer is subsequently formed between them, by the agency of one or both of them, and is called the mesoblast. From these three simple membranes, which are composed exclusively of cells, are formed all the complex tissues and organs of the adult man. The epiblast develops into the nervous system (brain, spinal cord, and nerves), and the cuticle, hair, etc. The hypoblast develops into the cellular parts of the digestive canal, the liver, lungs, etc. The mesoblast develops into the muscles, bones, ligaments, blood-vessels, etc. To trace the details of this evolution of a human being from the microscopic oösperm is a fascinating and instructive study, but is beyond the limits and purpose of this little book. It will well repay further careful study by the reader.
A careful study of the life-histories of the few unicellular animals and plants mentioned in the preceding pages will help to let one realize the wonderful powers of nucleated pieces of protoplasm (cells). If isolated pieces of protoplasm can accomplish so much, one will not be astonished to learn that many diverse cells associated together in intimate correlations, as occurs in the higher animals, may accomplish results that are profoundly interesting and marvelous. As far as anatomy and physiology alone can reveal, it is the result of millions of cells acting together that makes possible the existence of such a living, sentient, thinking creature as man or the highly intelligent elephant or any other multicellular animal. It is due to the mysterious powers of protoplasm that one little microscopic cell, like the fertilized ovum of a woman, is able to hold all the heritages of the race, and gradually unfold them as it builds up the body into myriads of diverse cells intimately associated together.
All of the wonderful results of Embryology are accomplished through cell-multiplications, cell-differentiations, cell-associations, invagination and evaginations of cell-groups (tissues or organs), and unequal growth of parts (cells or groups of cells).
Fig. 10.—Segmentation of the fertilized ovum and Gastrulation: 4, morula; 5, section through blastula showing hollow sphere; 6, gastrula showing outer layer of cells (epiblast) and inner layer (hypoblast); the 6 is at the mouth of the cavity (enteron) of the gastrula.
In concluding this brief but, we hope, useful study of a few selected cells, we may say that an eminent English physiologist has made the statement that a student who has not looked through the microscope and observed the circulation of the blood in the web of a frog’s foot is not fit to study medicine. However beautiful, fascinating and instructive the sight of this circulation may be, we are tempted to make the assertion that the student who has not looked through the microscope at some of the superficial ooze from the bottom of any slow-running stream, in summer, and observed the structure and actions of that wonderful little unicellular animal, the Amœba proteus, is still less prepared to study medicine. In this little speck of protoplasm the problems of life are reduced to their simplest forms, for all higher plants and animals may be regarded as groups of more or less modified amœbæ peculiarly associated together. In our studies of the amœba we will be forcibly reminded of a very clever trick which is practiced in India and is called the mango-trick. In this trick a seed is put into the ground and covered up, and after divers incantations a full-blown mango-bush appears within five minutes. We have never met any one who knew how this thing was done, nor have we ever seen a person who believed it to be anything else than a conjuring trick. So it is with the amœba, a beautiful and fascinating trickster of nature. We understand some of its activities, interesting and exceedingly instructive, but there are many others beyond our ken. We see the commencement and ending of many of its chemical activities, but there are numerous other intermediary processes that take place in the hidden recesses of the protoplasm, and concerning which we know nothing. It may fall to the lot of some reader of this little book, as a patient and keen observer, to unravel some of these mango-like tricks of the amœba or other unicellular creature.