When the mulberry-like cl.u.s.ter of cells has been formed, the border-cells of the lens separate from the rest and travel into the yelk and the border-layer. From this the blastula is developed; the regular bi-convex lens being converted into a disk, like a watch-gla.s.s, with thick borders. This lies on the upper and less curved polar surface of the nutritive yelk like the watch gla.s.s on the yelk. Fluid gathers between the outer layer and the border, and the segmentation-cavity is formed. The gastrula is then formed by inv.a.g.i.n.ation, or a kind of turning-up of the edge of the blastoderm.

In this process the segmentation-cavity disappears.

The s.p.a.ce underneath the entoderm corresponds to the primitive gut-cavity, and is filled with the decreasing food-yelk (n). Thus the formation of the gastrula of our fish is complete. In contrast to the two chief forms of gastrula we considered previously, we give the name of discoid gastrula (discogastrula, Figure 1.54) to this third princ.i.p.al type.

Very similar to the discoid gastrulation of the bony fishes is that of the hags or myxinoida, the remarkable cyclostomes that live parasitically in the body-cavity of fishes, and are distinguished by several notable peculiarities from their nearest relatives, the lampreys. While the amphiblastic ova of the latter are small and develop like those of the amphibia, the cuc.u.mber-shaped ova of the hag are about an inch long, and form a discoid gastrula. Up to the present it has only been observed in one species (Bdellostoma Stouti), by Dean and Doflein (1898).

It is clear that the important features which distinguish the discoid gastrula from the other chief forms we have considered are determined by the large food-yelk. This takes no direct part in the building of the germinal layers, and completely fills the primitive gut-cavity of the gastrula, even protruding at the mouth-opening. If we imagine the original bell-gastrula (Figures 1.30 to 1.36) trying to swallow a ball of food which is much bigger than itself, it would spread out round it in discoid shape in the attempt, just as we find to be the case here (Figure 1.54). Hence we may derive the discoid gastrula from the original bell-gastrula, through the intermediate stage of the hooded gastrula. It has arisen through the acc.u.mulation of a store of food-stuff at the vegetal pole, a "nutritive yelk" being thus formed in contrast to the "formative yelk." Nevertheless, the gastrula is formed here, as in the previous cases, by the folding or inv.a.g.i.n.ation of the blastula. We can, therefore, reduce this cenogenetic form of the discoid segmentation to the palingenetic form of the primitive cleavage.

(FIGURE 1.53. Ovum-segmentation of a bony fish. A first cleavage of the stem-cell (cytula), B division of same into four segmentation-cells (only two visible), C the germinal disk divides into the blastoderm (b) and the periblast (p). d nutritive yelk, f fat-globule, c ovolemma, z s.p.a.ce between the ovolemma and the ovum, filled with a clear fluid.)

This reduction is tolerably easy and confident in the case of the small ovum of our deep-sea bony fish, but it becomes difficult and uncertain in the case of the large ova that we find in the majority of the other fishes and in all the reptiles and birds. In these cases the food-yelk is, in the first place, comparatively colossal, the formative yelk being almost invisible beside it; and, in the second place, the food-yelk contains a quant.i.ty of different elements, which are known as "yelk-granules, yelk-globules, yelk-plates, yelk-flakes, yelk-vesicles," and so on. Frequently these definite elements in the yelk have been described as real cells, and it has been wrongly stated that a portion of the embryonic body is built up from these cells.

This is by no means the case. In every case, however large it is--and even when cell-nuclei travel into it during the cleavage of the border--the nutritive yelk remains a dead acc.u.mulation of food, which is taken into the gut during embryonic development and consumed by the embryo. The latter develops solely from the living formative yelk of the stem-cell. This is equally true of the ova of our small bony fishes and of the colossal ova of the primitive fishes, reptiles, and birds.

(FIGURE 1.54. Discoid gastrula (discogastrula) of a bony fish. e ectoderm, i entoderm, w border-swelling or primitive mouth, n alb.u.minous globule of the nutritive yelk, f fat-globule of same, c external membrane (ovolemma), d part.i.tion between entoderm and ectoderm (earlier the segmentation-cavity).)

The gastrulation of the primitive fishes or selachii (sharks and rays) has been carefully studied of late years by Ruckert, Rabl, and H.E.

Ziegler in particular, and is very important in the sense that this group is the oldest among living fishes, and their gastrulation can be derived directly from that of the cyclostoma by the acc.u.mulation of a large quant.i.ty of food-yelk. The oldest sharks (Cestracion) still have the unequal segmentation inherited from the cyclostoma. But while in this case, as in the case of the amphibia, the small ovum completely divides into cells in segmentation, this is no longer so in the great majority of the selachii (or Elasmobranchii). In these the contractility of the active protoplasm no longer suffices to break up the huge ma.s.s of the pa.s.sive deutoplasm completely into cells; this is only possible in the upper or dorsal part, but not in the lower or ventral section. Hence we find in the primitive fishes a blastula with a small eccentric segmentation-cavity (Figure 1.55 b), the wall of which varies greatly in composition. The circular border of the germinal disk which connects the roof and floor of the segmentation-cavity corresponds to the border-zone at the equator of the amphibian ovum. In the middle of its hinder border we have the beginning of the inv.a.g.i.n.ation of the primitive gut (Figure 1.56 ud); it extends gradually from this spot (which corresponds to the Rusconian a.n.u.s of the amphibia) forward and around, so that the primitive mouth becomes first crescent-shaped and then circular, and, as it opens wider, surrounds the ball of the larger food-yelk.

Essentially different from the wide-mouthed discoid gastrula of most of the selachii is the narrow-mouthed discoid gastrula (or epigastrula) of the amniotes, the reptiles, birds, and monotremes; between the two--as an intermediate stage--we have the amphigastrula of the amphibia. The latter has developed from the amphigastrula of the ganoids and dipneusts, whereas the discoid amniote gastrula has been evolved from the amphibian gastrula by the addition of food-yelk.

This change of gastrulation is still found in the remarkable ophidia (Gymnophiona, Coecilia, or Peromela), serpent-like amphibia that live in moist soil in the tropics, and in many respects represent the transition from the gill-breathing amphibia to the lung-breathing reptiles. Their embryonic development has been explained by the fine studies of the brothers Sarasin of Ichthyophis glutinosa at Ceylon (1887), and those of August Brauer of the Hypogeophis rostrata in the Seych.e.l.les (1897). It is only by the historical and comparative study of these that we can understand the difficult and obscure gastrulation of the amniotes.

The bird's egg is particularly important for our purpose, because most of the chief studies of the development of the vertebrates are based on observations of the hen's egg during hatching. The mammal ovum is much more difficult to obtain and study, and for this practical and obvious reason very rarely thoroughly investigated. But we can get hens' eggs in any quant.i.ty at any time, and, by means of artificial incubation, follow the development of the embryo step by step. The bird's egg differs considerably from the tiny mammal ovum in size, a large quant.i.ty of food-yelk acc.u.mulating within the original yelk or the protoplasm of the ovum. This is the yellow ball which we commonly call the yolk of the egg. In order to understand the bird's egg aright--for it is very often quite wrongly explained--we must examine it in its original condition, and follow it from the very beginning of its development in the bird's ovary. We then see that the original ovum is a quite small, naked, and simple cell with a nucleus, not differing in either size or shape from the original ovum of the mammals and other animals (cf. Figure 1.13 E). As in the case of all the craniota (animals with a skull), the original or primitive ovum (protovum) is covered with a continuous layer of small cells. This membrane is the follicle, from which the ovum afterwards issues.

Immediately underneath it the structureless yelk-membrane is secreted from the yelk.

(FIGURE 1.55. Longitudinal section through the blastula of a shark (Pristiuris). (From Ruckert.) (Looked at from the left; to the right is the hinder end, H, to the left the fore end, V.) B segmentation-cavity, kz cells of the germinal membrane, dk yelk-nuclei.

FIGURE 1.56. Longitudinal section of the blastula of a shark (Pristiurus) at the beginning of gastrulation. (From Ruckert.) (Seen from the left.) V fore end, H hind end, B segmentation-cavity, ud first trace of the primitive gut, dk yelk-nuclei, fd fine-grained yelk, gd coa.r.s.e-grained yelk.)

The small primitive ovum of the bird begins very early to take up into itself a quant.i.ty of food-stuff through the yelk-membrane, and work it up into the "yellow yelk." In this way the ovum enters on its second stage (the metovum), which is many times larger than the first, but still only a single enlarged cell. Through the acc.u.mulation of the store of yellow yelk within the ball of protoplasm the nucleus it contains (the germinal vesicle) is forced to the surface of the ball.

Here it is surrounded by a small quant.i.ty of protoplasm, and with this forms the lens-shaped formative yelk (Figure 1.15 b). This is seen on the yellow yelk-ball, at a certain point of the surface, as a small round white spot--the "tread" (cicatricula). From this point a thread-like column of white nutritive yelk (d), which contains no yellow yelk-granules, and is softer than the yellow food-yelk, proceeds to the middle of the yellow yelk-ball, and forms there a small central globule of white yelk (Figure 1.15 d). The whole of this white yelk is not sharply separated from the yellow yelk, which shows a slight trace of concentric layers in the hard-boiled egg (Figure 1.15 c). We also find in the hen's egg, when we break the sh.e.l.l and take out the yelk, a round small white disk at its surface which corresponds to the tread. But this small white "germinal disk" is now further developed, and is really the gastrula of the chick. The body of the chick is formed from it alone. The whole white and yellow yelk-ma.s.s is without any significance for the formation of the embryo, it being merely used as food by the developing chick. The clear, glarous ma.s.s of alb.u.min that surrounds the yellow yelk of the bird's egg, and also the hard chalky sh.e.l.l, are only formed within the oviduct round the impregnated ovum.

When the fertilisation of the bird's ovum has taken place within the mother's body, we find in the lens-shaped stem-cell the progress of flat, discoid segmentation (Figure 1.57). First two equal segmentation-cells (A) are formed from the ovum. These divide into four (B), then into eight, sixteen (C), thirty-two, sixty-four, and so on. The cleavage of the cells is always preceded by a division of their nuclei. The cleavage surfaces between the segmentation-cells appear at the free surface of the tread as clefts. The first two divisions are vertical to each other, in the form of a cross (B). Then there are two more divisions, which cut the former at an angle of forty-five degrees. The tread, which thus becomes the germinal disk, now has the appearance of an eight-rayed star. A circular cleavage next taking place round the middle, the eight triangular cells divide into sixteen, of which eight are in the middle and eight distributed around (C). Afterwards circular clefts and radial clefts, directed towards the centre, alternate more or less irregularly (D, E). In most of the amniotes the formation of concentric and radial clefts is irregular from the very first; and so also in the hen's egg. But the final outcome of the cleavage-process is once more the formation of a large number of small cells of a similar nature. As in the case of the fish-ovum, these segmentation-cells form a round, lens-shaped disk, which corresponds to the morula, and is embedded in a small depression of the white yelk. Between the lens-shaped disk of the morula-cells and the underlying white yelk a small cavity is now formed by the acc.u.mulation of fluid, as in the fishes. Thus we get the peculiar and not easily recognisable blastula of the bird (Figure 1.58). The small segmentation-cavity (fh) is very flat and much compressed. The upper or dorsal wall (dw) is formed of a single layer of clear, distinctly separated cells; this corresponds to the upper or animal hemisphere of the triton-blastula (Figure 1.45). The lower or ventral wall of the flat dividing s.p.a.ce (vw) is made up of larger and darker segmentation-cells; it corresponds to the lower or vegetal hemisphere of the blastula of the water-salamander (Figure 1.45 dz). The nuclei of the yelk-cells, which are in this case especially numerous at the edge of the lens-shaped blastula, travel into the white yelk, increase by cleavage, and contribute even to the further growth of the germinal disk by furnishing it with food-stuff.

(FIGURE 1.57. Diagram of discoid segmentation in the bird's ovum (magnified about ten times). Only the formative yelk (the tread) is shown in these six figures (A to F), because cleavage only takes place in this. The much larger food-yelk, which does not share in the cleavage, is left out and merely indicated by the dark ring without.)

The inv.a.g.i.n.ation or the folding inwards of the bird-blastula takes place in this case also at the hinder pole of the subsequent chief axis, in the middle of the hind border of the round germinal disk (Figure 1.59 s). At this spot we have the most brisk cleavage of the cells; hence the cells are more numerous and smaller here than in the fore-half of the germinal disk. The border-swelling or thick edge of the disk is less clear but whiter behind, and is more sharply separated from contiguous parts. In the middle of its hind border there is a white, crescent-shaped groove--Koller's sickle-groove (Fig 1.59 s); a small projecting process in the centre of it is called the sickle-k.n.o.b (sk). This important cleft is the primitive mouth, which was described for a long time as the "primitive groove." If we make a vertical section through this part, we see that a flat and broad cleft stretches under the germinal disk forwards from the primitive mouth; this is the primitive gut (Figure 1.60 ud). Its roof or dorsal wall is formed by the folded upper part of the blastula, and its floor or ventral wall by the white yelk (wd), in which a number of yelk-nuclei (dk) are distributed. There is a brisk multiplication of these at the edge of the germinal disk, especially in the neighbourhood of the sickle-shaped primitive mouth.

We learn from sections through later stages of this discoid bird-gastrula that the primitive gut-cavity, extending forward from the primitive mouth as a flat pouch, undermines the whole region of the round flat lens-shaped blastula (Figure 1.61 ud). At the same time, the segmentation-cavity gradually disappears altogether, the folded inner germinal layer (ik) placing itself from underneath on the overlying outer germinal layer (ak). The typical process of inv.a.g.i.n.ation, though greatly disguised, can thus be clearly seen in this case, as Goette and Rauber, and more recently Duval (Figure 1.61), have shown.

(FIGURE 1.58. Vertical section of the blastula of a hen (discoblastula). fh segmentation-cavity, dw dorsal wall of same, vw ventral wall, pa.s.sing directly into the white yelk (wd) (From Duval.)

FIGURE 1.59. The germinal disk of the hen's ovum at the beginning of gastrulation; A before incubation, B in the first hour of incubation. (From Koller.) ks germinal-disk, V its fore and H its hind border; es embryonic shield, s sickle-groove, sk sickle k.n.o.b, d yelk.

FIGURE 1.60. Longitudinal section of the germinal disk of a siskin (discogastrula). (From Duval.) ud primitive gut, vl, hl fore and hind lips of the primitive mouth (or sickle-edge); ak outer germinal layer, ik inner germinal layer, dk yelk-nuclei, wd white yelk.

FIGURE 1.61. Longitudinal section of the discoid gastrula of the nightingale. (From Duval.) ud primitive gut, vl, hl fore and hind lips of the primitive mouth; ak, ik outer and inner germinal layers; vr fore-border of the discogastrula.)

The older embryologists (Pander, Baer, Remak), and, in recent times especially, His, Kolliker, and others, said that the two primary germinal layers of the hen's ovum--the oldest and most frequent subject of observation!--arose by horizontal cleavage of a simple germinal disk. In opposition to this accepted view, I affirmed in my Gastraea Theory (1873) that the discoid bird-gastrula, like that of all other vertebrates, is formed by folding (or inv.a.g.i.n.ation), and that this typical process is merely altered in a peculiar way and disguised by the immense acc.u.mulation of food-yelk and the flat spreading of the discoid blastula at one part of its surface. I endeavoured to establish this view by the derivation of the vertebrates from one source, and especially by proving that the birds descend from the reptiles, and these from the amphibia. If this is correct, the discoid gastrula of the amniotes must have been formed by the folding-in of a hollow blastula, as has been shown by Remak and Rusconi of the discoid gastrula of the amphibia, their direct ancestors. The accurate and extremely careful observations of the authors I have mentioned (Goette, Rauber, and Duval) have decisively proved this recently for the birds; and the same has been done for the reptiles by the fine studies of Kupffer, Beneke, Wenkebach, and others. In the shield-shaped germinal disk of the lizard (Figure 1.62), the crocodile, the tortoise, and other reptiles, we find in the middle of the hind border (at the same spot as the sickle groove in the bird) a transverse furrow (u), which leads into a flat, pouch-like, blind sac, the primitive gut. The fore (dorsal) and hind (ventral) lips of the transverse furrow correspond exactly to the lips of the primitive mouth (or sickle-groove) in the birds.

(FIGURE 1.62. Germinal disk of the lizard (Lacerta agilis). (From Kupffer.) u primitive mouth, s sickle, es embryonic shield, hf and df light and dark germinative area.)

The gastrulation of the mammals must be derived from this special embryonic development of the reptiles and birds. This latest and most advanced cla.s.s of the vertebrates has, as we shall see afterwards, evolved at a comparatively recent date from an older group of reptiles; and all these amniotes must have come originally from a common stem-form. Hence the distinctive embryonic process of the mammal must have arisen by cenogenetic modifications from the older form of gastrulation of the reptiles and birds. Until we admit this thesis we cannot understand the formation of the germinal layers in the mammal, and therefore in man.

I first advanced this fundamental principle in my essay On the Gastrulation of Mammals (1877), and sought to show in this way that I a.s.sumed a gradual degeneration of the food-yelk and the yelk-sac on the way from the proreptiles to the mammals. "The cenogenetic process of adaptation," I said, "which has occasioned the atrophy of the rudimentary yelk-sac of the mammal, is perfectly clear. It is due to the fact that the young of the mammal, whose ancestors were certainly oviparous, now remain a long time in the womb. As the great store of food-yelk, which the oviparous ancestors gave to the egg, became superfluous in their descendants owing to the long carrying in the womb, and the maternal blood in the wall of the uterus made itself the chief source of nourishment, the now useless yelk-sac was bound to atrophy by embryonic adaptation."

My opinion met with little approval at the time; it was vehemently attacked by Kolliker, Hensen, and His in particular. However, it has been gradually accepted, and has recently been firmly established by a large number of excellent studies of mammal gastrulation, especially by Edward Van Beneden's studies of the rabbit and bat, Selenka's on the marsupials and rodents, Heape's and Lieberkuhn's on the mole, Kupffer and Keibel's on the rodents, Bonnet's on the ruminants, etc.

From the general comparative point of view, Carl Rabl in his theory of the mesoderm, Oscar Hertwig in the latest edition of his Manual (1902), and Hubrecht in his Studies in Mammalian Embryology (1891), have supported the opinion, and sought to derive the peculiarly modified gastrulation of the mammal from that of the reptile.

(FIGURE 1.63. Ovum of the opossum (Didelphys) divided into four. (From Selenka.) b the four segmentation-cells, r directive body, c unnucleated coagulated matter, p, alb.u.min-membrane.)

In the meantime (1884) the studies of Wilhelm Haacke and Caldwell provided a proof of the long-suspected and very interesting fact, that the lowest mammals, the monotremes, LAY EGGS, like the birds and reptiles, and are not viviparous like the other mammals. Although the gastrulation of the monotremes was not really known until studied by Richard Semon in 1894, there could be little doubt, in view of the great size of their food-yelk, that their ovum-segmentation was discoid, and led to the formation of a sickle-mouthed discogastrula, as in the case of the reptiles and birds. Hence I had, in 1875 (in my essay on The Gastrula and Ovum-segmentation of Animals), counted the monotremes among the discoblastic vertebrates. This hypothesis was established as a fact nineteen years afterwards by the careful observations of Semon; he gave in the second volume of his great work, Zoological Journeys in Australia (1894), the first description and correct explanation of the discoid gastrulation of the monotremes. The fertilised ova of the two living monotremes (Echidna and Ornithorhynchus) are b.a.l.l.s of one-fifth of an inch in diameter, enclosed in a stiff sh.e.l.l; but they grow considerably during development, so that when laid the egg is three times as large. The structure of the plentiful yelk, and especially the relation of the yellow and the white yelk, are just the same as in the reptiles and birds. As with these, partial cleavage takes place at a spot on the surface at which the small formative yelk and the nucleus it encloses are found. First is formed a lens-shaped circular germinal disk. This is made up of several strata of cells, but it spreads over the yelk-ball, and thus becomes a one-layered blastula. If we then imagine the yelk it contains to be dissolved and replaced by a clear liquid, we have the characteristic blastula of the higher mammals. In these the gastrulation proceeds in two phases, as Semon rightly observes: firstly, formation of the entoderm by cleavage at the centre and further growth at the edge; secondly, inv.a.g.i.n.ation. In the monotremes more primitive conditions have been retained better than in the reptiles and birds. In the latter, before the commencement of the gastrula-folding, we have, at least at the periphery, a two-layered embryo forming from the cleavage. But in the monotremes the formation of the cenogenetic entoderm does not precede the inv.a.g.i.n.ation; hence in this case the construction of the germinal layers is less modified than in the other amniota.

The marsupials, a second sub-cla.s.s, come next to the oviparous monotremes, the oldest of the mammals. But as in their case the food-yelk is already atrophied, and the little ovum develops within the mother's body, the partial cleavage has been reconverted into total. One section of the marsupials still show points of agreement with the monotremes, while another section of them, according to the splendid investigations of Selenka, form a connecting-link between these and the placentals.

(FIGURE 1.64. Blastula of the opossum (Didelphys). (From Selenka.) a animal pole of the blastula, v vegetal pole, en mother-cell of the entoderm, ex ectodermic cells, s spermia, ib unnucleated yelk-b.a.l.l.s (remainder of the food-yelk), p alb.u.min membrane.)

The fertilised ovum of the opossum (Didelphys) divides, according to Selenka, first into two, then four, then eight equal cells; hence the segmentation is at first equal or h.o.m.ogeneous. But in the course of the cleavage a larger cell, distinguished by its less clear plasm and its containing more yelk-granules (the mother cell of the entoderm, Figure 1.64 en), separates from the others; the latter multiply more rapidly than the former. As, further, a quant.i.ty of fluid gathers in the morula, we get a round blastula, the wall of which is of varying thickness, like that of the amphioxus (Figure 1.38 E) and the amphibia (Figure 1.45). The upper or animal hemisphere is formed of a large number of small cells; the lower or vegetal hemisphere of a small number of large cells. One of the latter, distinguished by its size (Figure 1.64 en), lies at the vegetal pole of the blastula-axis, at the point where the primitive mouth afterwards appears. This is the mother-cell of the entoderm; it now begins to multiply by cleavage, and the daughter-cells (Figure 1.65 i) spread out from this spot over the inner surface of the blastula, though at first only over the vegetal hemisphere. The less clear entodermic cells (i) are distinguished at first by their rounder shape and darker nuclei from the higher, clearer, and longer entodermic cells (e), afterwards both are greatly flattened, the inner blastodermic cells more than the outer.

(FIGURE 1.65. Blastula of the opossum (Didelphys) at the beginning of gastrulation. (From Selenka.) e ectoderm, i entoderm; a animal pole, u primitive mouth at the vegetal pole, f segmentation-cavity, d unnucleated yelk-b.a.l.l.s (relics of the reduced food-yelk), c nucleated curd (without yelk-granules).

FIGURE 1.66. Oval gastrula of the opossum (Didelphys), about eight hours old. (From Selenka) (external view).)

The unnucleated yelk-b.a.l.l.s and curd (Figure 1.65 d) that we find in the fluid of the blastula in these marsupials are very remarkable; they are the relics of the atrophied food-yelk, which was developed in their ancestors, the monotremes, and in the reptiles.

In the further course of the gastrulation of the opossum the oval shape of the gastrula (Figure 1.66) gradually changes into globular, a larger quant.i.ty of fluid acc.u.mulating in the vesicle. At the same time, the entoderm spreads further and further over the inner surface of the ectoderm (e). A globular vesicle is formed, the wall of which consists of two thin simple strata of cells; the cells of the outer germinal layer are rounder, and those of the inner layer flatter. In the region of the primitive mouth (p) the cells are less flattened, and multiply briskly. From this point--from the hind (ventral) lip of the primitive mouth, which extends in a central cleft, the primitive groove--the construction of the mesoderm proceeds.

Gastrulation is still more modified and curtailed cenogenetically in the placentals than in the marsupials. It was first accurately known to us by the distinguished investigations of Edward Van Beneden in 1875, the first object of study being the ovum of the rabbit. But as man also belongs to this sub-cla.s.s, and as his as yet unstudied gastrulation cannot be materially different from that of the other placentals, it merits the closest attention. We have, in the first place, the peculiar feature that the two first segmentation-cells that proceed from the cleavage of the fertilised ovum (Figure 1.68) are of different sizes and natures; the difference is sometimes greater, sometimes less (Figure 1.69). One of these first daughter-cells of the ovum is a little larger, clearer, and more transparent than the other.

Further, the smaller cell takes a colour in carmine, osmium, etc., more strongly than the larger. By repeated cleavage of it a morula is formed, and from this a blastula, which changes in a very characteristic way into the greatly modified gastrula. When the number of the segmentation-cells in the mammal embryo has reached ninety-six (in the rabbit, about seventy hours after impregnation) the foetus a.s.sumes a form very like the archigastrula (Figure 1.72). The spherical embryo consists of a central ma.s.s of thirty-two soft, round cells with dark nuclei, which are flattened into polygonal shape by mutual pressure, and colour dark-brown with osmic acid (Figure 1.72 i). This dark central group of cells is surrounded by a lighter spherical membrane, consisting of sixty-four cube-shaped, small, and fine-grained cells which lie close together in a single stratum, and only colour slightly in osmic acid (Figure 1.72 e). The authors who regard this embryonic form as the primary gastrula of the placental conceive the outer layer as the ectoderm and the inner as the entoderm. The entodermic membrane is only interrupted at one spot, one, two, or three of the ectodermic cells being loose there. These form the yelk-stopper, and fill up the mouth of the gastrula (a). The central primitive gut-cavity (d) is full of entodermic cells. The uni-axial type of the mammal gastrula is accentuated in this way.

However, opinions still differ considerably as to the real nature of this "provisional gastrula" of the placental and its relation to the blastula into which it is converted.

As the gastrulation proceeds a large spherical blastula is formed from this peculiar solid amphigastrula of the placental, as we saw in the case of the marsupial. The acc.u.mulation of fluid in the solid gastrula (Figure 1.73 A) leads to the formation of an eccentric cavity, the group of the darker entodermic cells (hy) remaining directly attached at one spot with the round enveloping stratum of the lighter ectodermic cells (ep). This spot corresponds to the original primitive mouth (prostoma or blastoporus). From this important spot the inner germinal layer spreads all round on the inner surface of the outer layer, the cell-stratum of which forms the wall of the hollow sphere; the extension proceeds from the vegetal towards the animal pole.

(FIGURE 1.67. Longitudinal section through the oval gastrula of the opossum (Figure 1.69). (From Selenka.) p primitive mouth, e ectoderm, i entoderm, d yelk remains in the primitive gut-cavity (u).)

The cenogenetic gastrulation of the placental has been greatly modified by secondary adaptation in the various groups of this most advanced and youngest sub-cla.s.s of the mammals. Thus, for instance, we find in many of the rodents (guinea-pigs, mice, etc.) APPARENTLY a temporary inversion of the two germinal layers. This is due to a folding of the blastodermic wall by what is called the "girder," a plug-shaped growth of Rauber's "roof-layer." It is a thin layer of flat epithelial cells, that is freed from the surface of the blastoderm in some of the rodents; it has no more significance in connection with the general course of placental gastrulation than the conspicuous departure from the usual globular shape in the blastula of some of the ungulates. In some pigs and ruminants it grows into a thread-like, long and thin tube.

(FIGURE 1.68. Stem-cell of the mammal ovum (from the rabbit). k stem-nucleus, n nuclear corpuscle, p protoplasm of the stem-cell, z modified zona pellucida, h outer alb.u.minous membrane, s dead sperm-cells.

FIGURE 1.69. Incipient cleavage of the mammal ovum (from the rabbit).

The stem-cell has divided into two unequal cells, one lighter (e) and one darker (i). z zona pellucida, h outer alb.u.minous membrane, s dead sperm-cell.

FIGURE 1.70. The first four segmentation-cells of the mammal ovum (from the rabbit). e the two larger (and lighter) cells, i the two smaller (and darker) cells, z zona pellucida, h outer alb.u.minous membrane.

FIGURE 1.71. Mammal ovum with eight segmentation-cells (from the rabbit). e four larger and lighter cells, i four smaller and darker cells, z zona pellucida, h outer alb.u.minous membrane.)

Thus the gastrulation of the placentals, which diverges most from that of the amphioxus, the primitive form, is reduced to the original type, the inv.a.g.i.n.ation of a modified blastula. Its chief peculiarity is that the folded part of the blastoderm does not form a completely closed (only open at the primitive mouth) blind sac, as is usual; but this blind sac has a wide opening at the ventral curve (opposite to the dorsal mouth); and through this opening the primitive gut communicates from the first with the embryonic cavity of the blastula. The folded crest-shaped entoderm grows with a free circular border on the inner surface of the entoderm towards the vegetal pole; when it has reached this, and the inner surface of the blastula is completely grown over, the primitive gut is closed. This remarkable direct transition of the primitive gut-cavity into the segmentation-cavity is explained simply by the a.s.sumption that in most of the mammals the yelk-ma.s.s, which is still possessed by the oldest forms of the cla.s.s (the monotremes) and their ancestors (the reptiles), is atrophied. This proves the essential unity of gastrulation in all the vertebrates, in spite of the striking differences in the various cla.s.ses.

In order to complete our consideration of the important processes of segmentation and gastrulation, we will, in conclusion, cast a brief glance at the fourth chief type--superficial segmentation. In the vertebrates this form is not found at all. But it plays the chief part in the large stem of the articulates--the insects, spiders, myriapods, and crabs. The distinctive form of gastrula that comes of it is the "vesicular gastrula" (Perigastrula).