The process of fertilisation by s.e.xual conception consists, therefore, essentially in the coalescence and fusing together of two different cells. The lively spermatozoon travels towards the ovum by its serpentine movements, and bores its way into the female cell (Figure 1.23). The nuclei of both s.e.xual cells, attracted by a certain "affinity," approach each other and melt into one.

The fertilised cell is quite another thing from the unfertilised cell.

For if we must regard the spermia as real cells no less than the ova, and the process of conception as a coalescence of the two, we must consider the resultant cell as a quite new and independent organism.

It bears in the cell and nuclear matter of the penetrating spermatozoon a part of the father's body, and in the protoplasm and caryoplasm of the ovum a part of the mother's body. This is clear from the fact that the child inherits many features from both parents. It inherits from the father by means of the spermatozoon, and from the mother by means of the ovum. The actual blending of the two cells produces a third cell, which is the germ of the child, or the new organism conceived. One may also say of this s.e.xual coalescence that the STEM-CELL IS A SIMPLE HERMAPHRODITE; it unites both s.e.xual substances in itself.

(FIGURE 1.23. The fertilisation of the ovum by the spermatozoon (of a mammal). One of the many thread-like, lively spermidia pierces through a fine pore-ca.n.a.l into the nuclear yelk. The nucleus of the ovum is invisible.

FIGURE 1.24. An impregnated echinoderm ovum, with small h.o.m.ogeneous nucleus (e k). (From Hertwig.))

I think it necessary to emphasise the fundamental importance of this simple, but often unappreciated, feature in order to have a correct and clear idea of conception. With that end, I have given a special name to the new cell from which the child develops, and which is generally loosely called "the fertilised ovum," or "the first segmentation sphere." I call it "the stem-cell" (cytula). The name "stem-cell" seems to me the simplest and most suitable, because all the other cells of the body are derived from it, and because it is, in the strictest sense, the stem-father and stem-mother of all the countless generations of cells of which the multicellular organism is to be composed. That complicated molecular movement of the protoplasm which we call "life" is, naturally, something quite different in this stem-cell from what we find in the two parent-cells, from the coalescence of which it has issued. THE LIFE OF THE STEM-CELL OR CYTULA IS THE PRODUCT OR RESULTANT OF THE PATERNAL LIFE-MOVEMENT THAT IS CONVEYED IN THE SPERMATOZOON AND THE MATERNAL LIFE-MOVEMENT THAT IS CONTRIBUTED BY THE OVUM.

The admirable work done by recent observers has shown that the individual development, in man and the other animals, commences with the formation of a simple "stem-cell" of this character, and that this then pa.s.ses, by repeated segmentation (or cleavage), into a cl.u.s.ter of cells, known as "the segmentation sphere" or "segmentation cells." The process is most clearly observed in the ova of the echinoderms (star-fishes, sea-urchins, etc.). The investigations of Oscar and Richard Hertwig were chiefly directed to these. The main results may be summed up as follows:--

Conception is preceded by certain preliminary changes, which are very necessary--in fact, usually indispensable--for its occurrence. They are comprised under the general heading of "Changes prior to impregnation." In these the original nucleus of the ovum, the germinal vesicle, is lost. Part of it is extruded, and part dissolved in the cell contents; only a very small part of it is left to form the basis of a fresh nucleus, the p.r.o.nucleus femininus. It is the latter alone that combines in conception with the invading nucleus of the fertilising spermatozoon (the p.r.o.nucleus masculinus).

The impregnation of the ovum commences with a decay of the germinal vesicle, or the original nucleus of the ovum (Figure 1.8). We have seen that this is in most unripe ova a large, transparent, round vesicle. This germinal vesicle contains a viscous fluid (the caryolymph). The firm nuclear frame (caryobasis) is formed of the enveloping membrane and a mesh-work of nuclear threads running across the interior, which is filled with the nuclear sap. In a knot of the network is contained the dark, stiff, opaque nuclear corpuscle or nucleolus. When the impregnation of the ovum sets in, the greater part of the germinal vesicle is dissolved in the cell; the nuclear membrane and mesh-work disappear; the nuclear sap is distributed in the protoplasm; a small portion of the nuclear base is extruded; another small portion is left, and is converted into the secondary nucleus, or the female pro-nucleus (Figure 1.24 e k).

The small portion of the nuclear base which is extruded from the impregnated ovum is known as the "directive bodies" or "polar cells"; there are many disputes as to their origin and significance, but we are as yet imperfectly acquainted with them. As a rule, they are two small round granules, of the same size and appearance as the remaining pro-nucleus. They are detached cell-buds; their separation from the large mother-cell takes place in the same way as in ordinary "indirect cell-division." Hence, the polar cells are probably to be conceived as "abortive ova," or "rudimentary ova," which proceed from a simple original ovum by cleavage in the same way that several sperm-cells arise from one "sperm-mother-cell," in reproduction from sperm. The male sperm-cells in the t.e.s.t.i.c.l.es must undergo similar changes in view of the coming impregnation as the ova in the female ovary. In this maturing of the sperm each of the original seed-cells divides by double segmentation into four daughter-cells, each furnished with a fourth of the original nuclear matter (the hereditary chromatin); and each of these four descendant cells becomes a spermatozoon, ready for impregnation. Thus is prevented the doubling of the chromatin in the coalescence of the two nuclei at conception. As the two polar cells are extruded and lost, and have no further part in the fertilisation of the ovum, we need not discuss them any further. But we must give more attention to the female pro-nucleus which alone remains after the extrusion of the polar cells and the dissolving of the germinal vesicle (Figure 1.23 e k). This tiny round corpuscle of chromatin now acts as a centre of attraction for the invading spermatozoon in the large ripe ovum, and coalesces with its "head," the male pro-nucleus.

The product of this blending, which is the most important part of the act of impregnation, is the stem-nucleus, or the first segmentation nucleus (archicaryon)--that is to say, the nucleus of the new-born embryonic stem-cell or "first segmentation cell." This stem-cell is the starting point of the subsequent embryonic processes.

Hertwig has shown that the tiny transparent ova of the echinoderms are the most convenient for following the details of this important process of impregnation. We can, in this case, easily and successfully accomplish artificial impregnation, and follow the formation of the stem-cell step by step within the s.p.a.ce of ten minutes. If we put ripe ova of the star-fish or sea-urchin in a watch gla.s.s with sea-water and add a drop of ripe sperm-fluid, we find each ovum impregnated within five minutes. Thousands of the fine, mobile ciliated cells, which we have described as "sperm-threads" (Figure 1.20), make their way to the ova, owing to a sort of chemical sensitive action which may be called "smell." But only one of these innumerable spermatozoa is chosen--namely, the one that first reaches the ovum by the serpentine motions of its tail, and touches the ovum with its head. At the spot where the point of its head touches the surface of the ovum the protoplasm of the latter is raised in the form of a small wart, the "impregnation rise" (Figure 1.25 A). The spermatozoon then bores its way into this with its head, the tail outside wriggling about all the time (Figure 1.25 B, C). Presently the tail also disappears within the ovum. At the same time the ovum secretes a thin external yelk-membrane (Figure 1.25 C), starting from the point of impregnation; and this prevents any more spermatozoa from entering.

Inside the impregnated ovum we now see a rapid series of most important changes. The pear-shaped head of the sperm-cell, or the "head of the spermatozoon," grows larger and rounder, and is converted into the male pro-nucleus (Figure 1.26 s k). This has an attractive influence on the fine granules or particles which are distributed in the protoplasm of the ovum; they arrange themselves in lines in the figure of a star. But the attraction or the "affinity" between the two nuclei is even stronger. They move towards each other inside the yelk with increasing speed, the male (Figure 1.27 s k) going more quickly than the female nucleus (e k). The tiny male nucleus takes with it the radiating mantle which spreads like a star about it. At last the two s.e.xual nuclei touch (usually in the centre of the globular ovum), lie close together, are flattened at the points of contact, and coalesce into a common ma.s.s. The small central particle of nuclein which is formed from this combination of the nuclei is the stem-nucleus, or the first segmentation nucleus; the new-formed cell, the product of the impregnation, is our stem-cell, or "first segmentation sphere" (Figure 1.2).

(FIGURE 1.25. Impregnation of the ovum of a star-fish. (From Hertwig.) Only a small part of the surface of the ovum is shown. One of the numerous spermatozoa approaches the "impregnation rise" (A), touches it (B), and then penetrates into the protoplasm of the ovum (C).

FIGURES 1.26 AND 1.27. Impregnation of the ovum of the sea-urchin.

(From Hertwig.) In Figure 1.26 the little sperm-nucleus (sk) moves towards the larger nucleus of the ovum (ek). In Figure 1.27 they nearly touch, and are surrounded by the radiating mantle of protoplasm.)

Hence the one essential point in the process of s.e.xual reproduction or impregnation is the formation of a new cell, the stem-cell, by the combination of two originally different cells, the female ovum and the male spermatozoon. This process is of the highest importance, and merits our closest attention; all that happens in the later development of this first cell and in the life of the organism that comes of it is determined from the first by the chemical and morphological composition of the stem-cell, its nucleus and its body.

We must, therefore, make a very careful study of the rise and structure of the stem-cell.

The first question that arises is as to the two different active elements, the nucleus and the protoplasm, in the actual coalescence.

It is obvious that the nucleus plays the more important part in this.

Hence Hertwig puts his theory of conception in the principle: "Conception consists in the copulation of two cell-nuclei, which come from a male and a female cell." And as the phenomenon of heredity is inseparably connected with the reproductive process, we may further conclude that these two copulating nuclei "convey the characteristics which are transmitted from parents to offspring." In this sense I had in 1866 (in the ninth chapter of the General Morphology) ascribed to the reproductive nucleus the function of generation and heredity, and to the nutritive protoplasm the duties of nutrition and adaptation.

As, moreover, there is a complete coalescence of the mutually attracted nuclear substances in conception, and the new nucleus formed (the stem-nucleus) is the real starting-point for the development of the fresh organism, the further conclusion may be drawn that the male nucleus conveys to the child the qualities of the father, and the female nucleus the features of the mother. We must not forget, however, that the protoplasmic bodies of the copulating cells also fuse together in the act of impregnation; the cell-body of the invading spermatozoon (the trunk and tail of the male ciliated cell) is dissolved in the yelk of the female ovum. This coalescence is not so important as that of the nuclei, but it must not be overlooked; and, though this process is not so well known to us, we see clearly at least the formation of the star-like figure (the radial arrangement of the particles in the plasma) in it (Figures 1.26 to 1.27).

The older theories of impregnation generally went astray in regarding the large ovum as the sole base of the new organism, and only ascribed to the spermatozoon the work of stimulating and originating its development. The stimulus which it gave to the ovum was sometimes thought to be purely chemical, at other times rather physical (on the principle of transferred movement), or again a mystic and transcendental process. This error was partly due to the imperfect knowledge at that time of the facts of impregnation, and partly to the striking difference in the sizes of the two s.e.xual cells. Most of the earlier observers thought that the spermatozoon did not penetrate into the ovum. And even when this had been demonstrated, the spermatozoon was believed to disappear in the ovum without leaving a trace.

However, the splendid research made in the last three decades with the finer technical methods of our time has completely exposed the error of this. It has been shown that the tiny sperm-cell is NOT SUBORDINATED TO, BUT COORDINATED WITH, the large ovum. The nuclei of the two cells, as the vehicles of the hereditary features of the parents, are of equal physiological importance. In some cases we have succeeded in proving that the ma.s.s of the active nuclear substance which combines in the copulation of the two s.e.xual nuclei is originally the same for both.

These morphological facts are in perfect harmony with the familiar physiological truth that the child inherits from both parents, and that on the average they are equally distributed. I say "on the average," because it is well known that a child may have a greater likeness to the father or to the mother; that goes without saying, as far as the primary s.e.xual characters (the s.e.xual glands) are concerned. But it is also possible that the determination of the latter--the weighty determination whether the child is to be a boy or a girl--depends on a slight qualitative or quant.i.tative difference in the nuclein or the coloured nuclear matter which comes from both parents in the act of conception.

The striking differences of the respective s.e.xual cells in size and shape, which occasioned the erroneous views of earlier scientists, are easily explained on the principle of division of labour. The inert, motionless ovum grows in size according to the quant.i.ty of provision it stores up in the form of nutritive yelk for the development of the germ. The active swimming sperm-cell is reduced in size in proportion to its need to seek the ovum and bore its way into its yelk. These differences are very conspicuous in the higher animals, but they are much less in the lower animals. In those protists (unicellular plants and animals) which have the first rudiments of s.e.xual reproduction the two copulating cells are at first quite equal. In these cases the act of impregnation is nothing more than a sudden GROWTH, in which the originally simple cell doubles its volume, and is thus prepared for reproduction (cell-division). Afterwards slight differences are seen in the size of the copulating cells; though the smaller ones still have the same shape as the larger ones. It is only when the difference in size is very p.r.o.nounced that a notable difference in shape is found: the sprightly sperm-cell changes more in shape and the ovum in size.

Quite in harmony with this new conception of the EQUIVALENCE OF THE TWO GONADS, or the equal physiological importance of the male and female s.e.x-cells and their equal share in the process of heredity, is the important fact established by Hertwig (1875), that in normal impregnation only one single spermatozoon copulates with one ovum; the membrane which is raised on the surface of the yelk immediately after one sperm-cell has penetrated (Figure 1.25 C) prevents any others from entering. All the rivals of the fortunate penetrator are excluded, and die without. But if the ovum pa.s.ses into a morbid state, if it is made stiff by a lowering of its temperature or stupefied with narcotics (chloroform, morphia, nicotine, etc.), two or more spermatozoa may penetrate into its yelk-body. We then witness polyspermism. The more Hertwig chloroformed the ovum, the more spermatozoa were able to bore their way into its unconscious body.

(FIGURE 1.28. Stem-cell of a rabbit, magnified 200 times. In the centre of the granular protoplasm of the fertilised ovum (d) is seen the little, bright stem-nucleus, z is the ovolemma, with a mucous membrane (h). s are dead spermatozoa.)

These remarkable facts of impregnation are also of the greatest interest in psychology, especially as regards the theory of the cell-soul, which I consider to be its chief foundation. The phenomena we have described can only be understood and explained by ascribing a certain lower degree of psychic activity to the s.e.xual principles.

They FEEL each other's proximity, and are drawn together by a SENSITIVE impulse (probably related to smell); they MOVE towards each other, and do not rest until they fuse together. Physiologists may say that it is only a question of a peculiar physico-chemical phenomenon, and not a psychic action; but the two cannot be separated. Even the psychic functions, in the strict sense of the word, are only complex physical processes, or "psycho-physical" phenomena, which are determined in all cases exclusively by the chemical composition of their material substratum.

The monistic view of the matter becomes clear enough when we remember the radical importance of impregnation as regards heredity. It is well known that not only the most delicate bodily structures, but also the subtlest traits of mind, are transmitted from the parents to the children. In this the chromatic matter of the male nucleus is just as important a vehicle as the large caryoplasmic substance of the female nucleus; the one transmits the mental features of the father, and the other those of the mother. The blending of the two parental nuclei determines the individual psychic character of the child.

But there is another important psychological question--the most important of all--that has been definitely answered by the recent discoveries in connection with conception. This is the question of the immortality of the soul. No fact throws more light on it and refutes it more convincingly than the elementary process of conception that we have described. For this copulation of the two s.e.xual nuclei (Figures 1.26 and 1.27) indicates the precise moment at which the individual begins to exist. All the bodily and mental features of the new-born child are the sum-total of the hereditary qualities which it has received in reproduction from parents and ancestors. All that man acquires afterwards in life by the exercise of his organs, the influence of his environment, and education--in a word, by adaptation--cannot obliterate that general outline of his being which he inherited from his parents. But this hereditary disposition, the essence of every human soul, is not "eternal," but "temporal"; it comes into being only at the moment when the sperm-nucleus of the father and the nucleus of the maternal ovum meet and fuse together. It is clearly irrational to a.s.sume an "eternal life without end" for an individual phenomenon, the commencement of which we can indicate to a moment by direct visual observation.

The great importance of the process of impregnation in answering such questions is quite clear. It is true that conception has never been studied microscopically in all its details in the human case--notwithstanding its occurrence at every moment--for reasons that are obvious enough. However, the two cells which need consideration, the female ovum and the male spermatozoon, proceed in the case of man in just the same way as in all the other mammals; the human foetus or embryo which results from copulation has the same form as with the other animals. Hence, no scientist who is acquainted with the facts doubts that the processes of impregnation are just the same in man as in the other animals.

The stem-cell which is produced, and with which every man begins his career, cannot be distinguished in appearance from those of other mammals, such as the rabbit (Figure 1.28). In the case of man, also, this stem-cell differs materially from the original ovum, both in regard to form (morphologically), in regard to material composition (chemically), and in regard to vital properties (physiologically). It comes partly from the father and partly from the mother. Hence it is not surprising that the child who is developed from it inherits from both parents. The vital movements of each of these cells form a sum of mechanical processes which in the last a.n.a.lysis are due to movements of the smallest vital parts, or the molecules, of the living substance. If we agree to call this active substance pla.s.son, and its molecules plastidules, we may say that the individual physiological character of each of these cells is due to its molecular plastidule-movement. HENCE, THE PLASTIDULE-MOVEMENT OF THE CYTULA IS THE RESULTANT OF THE COMBINED PLASTIDULE-MOVEMENTS OF THE FEMALE OVUM AND THE MALE SPERM-CELL.* (* The pla.s.son of the stem-cell or cytula may, from the anatomical point of view, be regarded as h.o.m.ogeneous and structureless, like that of the monera. This is not inconsistent with our hypothetical ascription to the plastidules (or molecules of the pla.s.son) of a complex molecular structure. The complexity of this is the greater in proportion to the complexity of the organism that is developed from it and the length of the chain of its ancestry, or to the mult.i.tude of antecedent processes of heredity and adaptation.)

CHAPTER 1.8. THE GASTRAEA THEORY.

There is a substantial agreement throughout the animal world in the first changes which follow the impregnation of the ovum and the formation of the stem-cell; they begin in all cases with the segmentation of the ovum and the formation of the germinal layers. The only exception is found in the protozoa, the very lowest and simplest forms of animal life; these remain unicellular throughout life. To this group belong the amoebae, gregarinae, rhizopods, infusoria, etc.

As their whole organism consists of a single cell, they can never form germinal layers, or definite strata of cells. But all the other animals--all the tissue-forming animals, or metazoa, as we call them, in contradistinction to the protozoa--construct real germinal layers by the repeated cleavage of the impregnated ovum. This we find in the lower cnidaria and worms, as well as in the more highly-developed molluscs, echinoderms, articulates, and vertebrates.

In all these metazoa, or multicellular animals, the chief embryonic processes are substantially alike, although they often seem to a superficial observer to differ considerably. The stem-cell that proceeds from the impregnated ovum always pa.s.ses by repeated cleavage into a number of simple cells. These cells are all direct descendants of the stem-cell, and are, for reasons we shall see presently, called segmentation-cells. The repeated cleavage of the stem-cell, which gives rise to these segmentation-spheres, has long been known as "segmentation." Sooner or later the segmentation-cells join together to form a round (at first, globular) embryonic sphere (blastula); they then form into two very different groups, and arrange themselves in two separate strata--the two primary germinal layers. These enclose a digestive cavity, the primitive gut, with an opening, the primitive mouth. We give the name of the gastrula to the important embryonic form that has these primitive organs, and the name of gastrulation to the formation of it. This ontogenetic process has a very great significance, and is the real starting-point of the construction of the multicellular animal body.

The fundamental embryonic processes of the cleavage of the ovum and the formation of the germinal layers have been very thoroughly studied in the last thirty years, and their real significance has been appreciated. They present a striking variety in the different groups, and it was no light task to prove their essential ident.i.ty in the whole animal world. But since I formulated the gastraea theory in 1872, and afterwards (1875) reduced all the various forms of segmentation and gastrulation to one fundamental type, their ident.i.ty may be said to have been established. We have thus mastered the law of unity which governs the first embryonic processes in all the animals.

Man is like all the other higher animals, especially the apes, in regard to these earliest and most important processes. As the human embryo does not essentially differ, even at a much later stage of development--when we already perceive the cerebral vesicles, the eyes, ears, gill-arches, etc.--from the similar forms of the other higher mammals, we may confidently a.s.sume that they agree in the earliest embryonic processes, segmentation and the formation of germinal layers. This has not yet, it is true, been established by observation.

We have never yet had occasion to dissect a woman immediately after impregnation and examine the stem-cell or the segmentation-cells in her oviduct. However, as the earliest human embryos we have examined, and the later and more developed forms, agree with those of the rabbit, dog, and other higher mammals, no reasonable man will doubt but that the segmentation and formation of layers are the same in both cases.

But the special form of segmentation and layer formation which we find in the mammal is by no means the original, simple, palingenetic form.

It has been much modified and cenogenetically altered by a very complex adaptation to embryonic conditions. We cannot, therefore, understand it altogether in itself. In order to do this, we have to make a COMPARATIVE study of segmentation and layer-formation in the animal world; and we have especially to seek the original, PALINGENETIC form from which the modified CENOGENETIC (see Chapter 1.1) form has gradually been developed.

This original unaltered form of segmentation and layer-formation is found to-day in only one case in the vertebrate-stem to which man belongs--the lowest and oldest member of the stem, the wonderful lancelet or amphioxus (cf. Chapters 2.16 and 2.17). But we find a precisely similar palingenetic form of embryonic development in the case of many of the invertebrate animals, as, for instance, the remarkable ascidia, the pond-snail (Limnaeus), and arrow-worm (Sagitta), and many of the echinoderms and cnidaria, such as the common star-fish and sea-urchin, many of the medusae and corals, and the simpler sponges (Olynthus). We may take as an ill.u.s.tration the palingenetic segmentation and germinal layer-formation in an eight-fold insular coral, which I discovered in the Red Sea, and described as Monoxenia Darwinii.

(FIGURE 1.29. Gastrulation of a coral (Monoxenia Darwinii). A, B, stem-cell (cytula) or impregnated ovum. In Figure A (immediately after impregnation) the nucleus is invisible. In Figure B (a little later) it is quite clear. C two segmentation-cells. D four segmentation-cells. E mulberry-formation (morula). F blastosphere (blastula). G blastula (transverse section). H depula, or hollowed blastula (transverse section). I gastrula (longitudinal section). K gastrula, or cup-sphere, external appearance.)

The impregnated ovum of this coral (Figure 1.29 A, B) first splits into two equal cells (C). First, the nucleus of the stem-cell and its central body divide into two halves. These recede from and repel each other, and act as centres of attraction on the surrounding protoplasm; in consequence of this, the protoplasm is constricted by a circular furrow, and, in turn, divides into two halves. Each of the two segmentation-cells thus produced splits in the same way into two equal cells. The four segmentation-cells (grand-daughters of the stem-cell) lie in one plane. Now, however, each of them subdivides into two equal halves, the cleavage of the nucleus again preceding that of the surrounding protoplasm. The eight cells which thus arise break into sixteen, these into thirty-two, and then (each being constantly halved) into sixty-four, 128, and so on.* (* The number of segmentation-cells thus produced increases geometrically in the original gastrulation, or the purest palingenetic form of cleavage.

However, in different animals the number reaches a different height, so that the morula, and also the blastula, may consist sometimes of thirty-two, sometimes of sixty-four, and sometimes of 128, or more, cells.) The final result of this repeated cleavage is the formation of a globular cl.u.s.ter of similar segmentation-cells, which we call the mulberry-formation or morula. The cells are thickly pressed together like the parts of a mulberry or blackberry, and this gives a lumpy appearance to the surface of the sphere (Figure E).* (* The segmentation-cells which make up the morula after the close of the palingenetic cleavage seem usually to be quite similar, and to present no differences as to size, form, and composition. That, however, does not prevent them from differentiating into animal and vegetative cells, even during the cleavage.)

When the cleavage is thus ended, the mulberry-like ma.s.s changes into a hollow globular sphere. Watery fluid or jelly gathers inside the globule; the segmentation-cells are loosened, and all rise to the surface. There they are flattened by mutual pressure, and a.s.sume the shape of truncated pyramids, and arrange themselves side by side in one regular layer (Figures F, G). This layer of cells is called the germinal membrane (or blastoderm); the h.o.m.ogeneous cells which compose its simple structure are called blastodermic cells; and the whole hollow sphere, the walls of which are made of the preceding, is called the blastula or blastosphere.* (* The blastula of the lower animals must not be confused with the very different blastula of the mammal, which is properly called the gastrocystis or blastocystis. This cenogenetic gastrocystis and the palingenetic blastula are sometimes very wrongly comprised under the common name of blastula or vesicula blastodermica.)

In the case of our coral, and of many other lower forms of animal life, the young embryo begins at once to move independently and swim about in the water. A fine, long, thread-like process, a sort of whip or lash, grows out of each blastodermic cell, and this independently executes vibratory movements, slow at first, but quicker after a time (Figure F). In this way each blastodermic cell becomes a ciliated cell. The combined force of all these vibrating lashes causes the whole blastula to move about in a rotatory fashion. In many other animals, especially those in which the embryo develops within enclosed membranes, the ciliated cells are only formed at a later stage, or even not formed at all. The blastosphere may grow and expand by the blastodermic cells (at the surface of the sphere) dividing and increasing, and more fluid is secreted in the internal cavity. There are still to-day some organisms that remain throughout life at the structural stage of the blastula--hollow vesicles that swim about by a ciliary movement in the water, the wall of which is composed of a single layer of cells, such as the volvox, the magosphaera, synura, etc. We shall speak further of the great phylogenetic significance of this fact in Chapter 2.19.

A very important and remarkable process now follows--namely, the curving or inv.a.g.i.n.ation of the blastula (Figure H). The vesicle with a single layer of cells for wall is converted into a cup with a wall of two layers of cells (cf. Figures G, H, I). A certain spot at the surface of the sphere is flattened, and then bent inward. This depression sinks deeper and deeper, growing at the cost of the internal cavity. The latter decreases as the hollow deepens. At last the internal cavity disappears altogether, the inner side of the blastoderm (that which lines the depression) coming to lie close on the outer side. At the same time, the cells of the two sections a.s.sume different sizes and shapes; the inner cells are more round and the outer more oval (Figure I). In this way the embryo takes the form of a cup or jar-shaped body, with a wall made up of two layers of cells, the inner cavity of which opens to the outside at one end (the spot where the depression was originally formed). We call this very important and interesting embryonic form the "cup-embryo" or "cup-larva" (gastrula, Figure 1.29, I longitudinal section, K external view). I have in my Natural History of Creation given the name of depula to the remarkable intermediate form which appears at the pa.s.sage of the blastula into the gastrula. In this intermediate stage there are two cavities in the embryo--the original cavity (blastocoel) which is disappearing, and the primitive gut-cavity (progaster) which is forming.

I regard the gastrula as the most important and significant embryonic form in the animal world. In all real animals (that is, excluding the unicellular protists) the segmentation of the ovum produces either a pure, primitive, palingenetic gastrula (Figure 1.29 I, K) or an equally instructive cenogenetic form, which has been developed in time from the first, and can be directly reduced to it. It is certainly a fact of the greatest interest and instructiveness that animals of the most different stems--vertebrates and tunicates, molluscs and articulates, echinoderms and annelids, cnidaria and sponges--proceed from one and the same embryonic form. In ill.u.s.tration I give a few pure gastrula forms from various groups of animals (Figures 1.30 to 1.35, explanation given below each).

(FIGURES 1.30 TO 1.35. In each figure d is the primitive-gut cavity, o primitive mouth, s segmentation-cavity, i entoderm (gut-layer), e ectoderm (skin layer).

FIGURE 1.30. (A) Gastrula of a very simple primitive-gut animal or gastraead (gastrophysema). (Haeckel.)

FIGURE 1.31. (B) Gastrula of a worm (Sagitta). (From Kowalevsky.)

FIGURE 1.32. (C) Gastrula of an echinoderm (star-fish, Uraster), not completely folded in (depula). (From Alexander Aga.s.siz.)

FIGURE 1.33. (D) Gastrula of an arthropod (primitive crab, Nauplius) (as 32).