The animal stems are indicated by the letters a-g: a Zoophyta. b Annelida. c Mollusca.

d Echinoderma. e Articulata. f Tunicata. g Vertebrata.

1. Total Segmentation. Holoblastic ova. Gastrula without separate food-yelk. Hologastrula.

1.1. Primitive Segmentation. Archiblastic ova. Bell-gastrula (archigastrula.) a. Many lower zoophyta (sponges, hydrapolyps, medusae, simpler corals).

b. Many lower annelids (sagitta, phoronis, many nematoda, etc., terebratula, argiope, pisidium).

c. Some lower molluscs.

d. Many echinoderms.

e. A few lower articulata (some brachiopods, copepods: Tardigrades, pteromalina).

f. Many tunicata.

g. The acrania (amphioxus).

1.2. Unequal Segmentation. Amphiblastic ova. Hooded-gastrula (amphigastrula).

a. Many zoophyta (sponges, medusae, corals, siphonophorae, ctenophora).

b. Most worms.

c. Most molluscs.

d. Many echinoderms (viviparous species and some others).

e. Some of the lower articulata (both crustacea and tracheata).

f. Many tunicata.

g. Cyclostoma, the oldest fishes, amphibia, mammals (not including man).

2. Partial Segmentation. Meroblastic ova. Gastrula with separate food-yelk. Merogastrula.

2.3. Discoid Segmentation. Discoblastic ova. Discoid gastrula.

c. Cephalopods or cuttlefish.

e. Many articulata, wood-lice, scorpions, etc.

g. Primitive fishes, bony fishes, reptiles, birds, monotremes.

2.4. Superficial Segmentation. Periblastic ova. Spherical-gastrula.

e. The great majority of the articulata (crustaceans, myriapods, arachnids, insects).

CHAPTER 1.9. THE GASTRULATION OF THE VERTEBRATE.*

(* Cf. Balfour's Manual of Comparative Embryology volume 2; Theodore Morgan's The Development of the Frog's Egg.)

The remarkable processes of gastrulation, ovum-segmentation, and formation of germinal layers present a most conspicuous variety. There is to-day only the lowest of the vertebrates, the amphioxus, that exhibits the original form of those processes, or the palingenetic gastrulation which we have considered in the preceding chapter, and which culminates in the formation of the archigastrula (Figure 1.38).

In all other extant vertebrates these fundamental processes have been more or less modified by adaptation to the conditions of embryonic development (especially by changes in the food-yelk); they exhibit various cenogenetic types of the formation of germinal layers.

However, the different cla.s.ses vary considerably from each other. In order to grasp the unity that underlies the manifold differences in these phenomena and their historical connection, it is necessary to bear in mind always the unity of the vertebrate-stem. This "phylogenetic unity," which I developed in my General Morphology in 1866, is now generally admitted. All impartial zoologists agree to-day that all the vertebrates, from the amphioxus and the fishes to the ape and man, descend from a common ancestor, "the primitive vertebrate."

Hence the embryonic processes, by which each individual vertebrate is developed, must also be capable of being reduced to one common type of embryonic development; and this primitive type is most certainly exhibited to-day by the amphioxus.

It must, therefore, be our next task to make a comparative study of the various forms of vertebrate gastrulation, and trace them backwards to that of the lancelet. Broadly speaking, they fall first into two groups: the older cyclostoma, the earliest fishes, most of the amphibia, and the viviparous mammals, have holoblastic ova--that is to say, ova with total, unequal segmentation; while the younger cyclostoma, most of the fishes, the cephalopods, reptiles, birds, and monotremes, have meroblastic ova, or ova with partial discoid segmentation. A closer study of them shows, however, that these two groups do not present a natural unity, and that the historical relations between their several divisions are very complicated. In order to understand them properly, we must first consider the various modifications of gastrulation in these cla.s.ses. We may begin with that of the amphibia.

The most suitable and most available objects of study in this cla.s.s are the eggs of our indigenous amphibia, the tailless frogs and toads, and the tailed salamander. In spring they are to be found in cl.u.s.ters in every pond, and careful examination of the ova with a lens is sufficient to show at least the external features of the segmentation.

In order to understand the whole process rightly and follow the formation of the germinal layers and the gastrula, the ova of the frog and salamander must be carefully hardened; then the thinnest possible sections must be made of the hardened ova with the microtome, and the tinted sections must be very closely compared under a powerful microscope.

The ova of the frog or toad are globular in shape, about the twelfth of an inch in diameter, and are cl.u.s.tered in jelly-like ma.s.ses, which are lumped together in the case of the frog, but form long strings in the case of the toad. When we examine the opaque, grey, brown, or blackish ova closely, we find that the upper half is darker than the lower. The middle of the upper half is in many species black, while the middle of the lower half is white.* (* The colouring of the eggs of the amphibia is caused by the acc.u.mulation of dark-colouring matter at the animal pole of the ovum. In consequence of this, the animal cells of the ectoderm are darker than the vegetal cells of the entoderm. We find the reverse of this in the case of most animals, the protoplasm of the entoderm cells being usually darker and coa.r.s.er-grained.) In this way we get a definite axis of the ovum with two poles. To give a clear idea of the segmentation of this ovum, it is best to compare it with a globe, on the surface of which are marked the various parallels of longitude and lat.i.tude. The superficial dividing lines between the different cells, which come from the repeated segmentation of the ovum, look like deep furrows on the surface, and hence the whole process has been given the name of furcation. In reality, however, this "furcation," which was formerly regarded as a very mysterious process, is nothing but the familiar, repeated cell-segmentation. Hence also the segmentation-cells which result from it are real cells.

(FIGURE 1.40. The cleavage of the frog's ovum (magnified ten times). A stem-cell. B the first two segmentation-cells. C four cells. D eight cells (4 animal and 4 vegetative). E twelve cells (8 animal and 4 vegetative). F sixteen cells (8 animal and 8 vegetative). G twenty-four cells (16 animal and 8 vegetative). H thirty-two cells. I forty-eight cells. K sixty-four cells. L ninety-six cells. M 160 cells (128 animal and 32 vegetative).

(FIGURES 1.41 TO 1.44. Four vertical sections of the fertilised ovum of the toad, in four successive stages of development. The letters have the same meaning throughout: F segmentation-cavity. D covering of same (D dorsal half of the embryo, P ventral half). P yelk-stopper (white round field at the lower pole). Z yelk-cells of the entoderm (Remak's "glandular embryo"). N primitive gut cavity (progaster or Rusconian alimentary cavity). The primitive mouth (prostoma) is closed by the yelk-stopper, P. s part.i.tion between the primitive gut cavity (N) and the segmentation cavity (F). k k apostrophe, section of the large circular lip-border of the primitive mouth (the Rusconian a.n.u.s).

The line of dots between k and k apostrophe indicates the earlier connection of the yelk-stopper (P) with the central ma.s.s of the yelk-cells (Z). In Figure 1.44 the ovum has turned 90 degrees, so that the back of the embryo is uppermost and the ventral side down. (From Stricker.)).

The unequal segmentation which we observe in the ovum of the amphibia has the special feature of beginning at the upper and darker pole (the north pole of the terrestrial globe in our ill.u.s.tration), and slowly advancing towards the lower and brighter pole (the south pole). Also the upper and darker hemisphere remains in this position throughout the course of the segmentation, and its cells multiply much more briskly. Hence the cells of the lower hemisphere are found to be larger and less numerous. The cleavage of the stem-cell (Figure 1.40 A) begins with the formation of a complete furrow, which starts from the north pole and reaches to the south (B). An hour later a second furrow arises in the same way, and this cuts the first at a right angle (Figure 1.40 C). The ovum is thus divided into four equal parts.

Each of these four "segmentation cells" has an upper and darker and a lower, brighter half. A few hours later a third furrow appears, vertically to the first two (Figure 1.40 D). The globular germ now consists of eight cells, four smaller ones above (northern) and four larger ones below (southern). Next, each of the four upper ones divides into two halves by a cleavage beginning from the north pole, so that we now have eight above and four below (Figure 1.40 E). Later, the four new longitudinal divisions extend gradually to the lower cells, and the number rises from twelve to sixteen (F). Then a second circular furrow appears, parallel to the first, and nearer to the north pole, so that we may compare it to the north polar circle. In this way we get twenty-four segmentation-cells--sixteen upper, smaller, and darker ones, and eight smaller and brighter ones below (G). Soon, however, the latter also sub-divide into sixteen, a third or "meridian of lat.i.tude" appearing, this time in the southern hemisphere: this makes thirty-two cells altogether (H). Then eight new longitudinal lines are formed at the north pole, and these proceed to divide, first the darker cells above and afterwards the lighter southern cells, and finally reach the south pole. In this way we get in succession forty, forty-eight, fifty-six, and at last sixty-four cells (I, K). In the meantime, the two hemispheres differ more and more from each other. Whereas the sluggish lower hemisphere long remains at thirty-two cells, the lively northern hemisphere briskly sub-divides twice, producing first sixty-four and then 128 cells (L, M). Thus we reach a stage in which we count on the surface of the ovum 128 small cells in the upper half and thirty-two large ones in the lower half, or 160 altogether. The dissimilarity of the two halves increases: while the northern breaks up into a great number of small cells, the southern consists of a much smaller number of larger cells.

Finally, the dark cells of the upper half grow almost over the surface of the ovum, leaving only a small circular spot at the south pole, where the large and clear cells of the lower half are visible. This white region at the south pole corresponds, as we shall see afterwards, to the primitive mouth of the gastrula. The whole ma.s.s of the inner and larger and clearer cells (including the white polar region) belongs to the entoderm or ventral layer. The outer envelope of dark smaller cells forms the ectoderm or skin-layer.

In the meantime, a large cavity, full of fluid, has been formed within the globular body--the segmentation-cavity or embryonic cavity (blastocoel, Figures 1.41 to 1.44 F). It extends considerably as the cleavage proceeds, and afterwards a.s.sumes an almost semi-circular form (Figure 1.41 F). The frog-embryo now represents a modified embryonic vesicle or blastula, with hollow animal half and solid vegetal half.

Now a second, narrower but longer, cavity arises by a process of folding at the lower pole, and by the falling away from each other of the white entoderm-cells (Figures 1.41 to 1.44 N). This is the primitive gut-cavity or the gastric cavity of the gastrula, progaster or archenteron. It was first observed in the ovum of the amphibia by Rusconi, and so called the Rusconian cavity. The reason of its peculiar narrowness here is that it is, for the most part, full of yelk-cells of the entoderm. These also stop up the whole of the wide opening of the primitive mouth, and form what is known as the "yelk-stopper," which is seen freely at the white round spot at the south pole (P). Around it the ectoderm is much thicker, and forms the border of the primitive mouth, the most important part of the embryo (Figure 1.44 k, k apostrophe). Soon the primitive gut-cavity stretches further and further at the expense of the segmentation-cavity (F), until at last the latter disappears altogether. The two cavities are only separated by a thin part.i.tion (Figure 1.43 s). With the formation of the primitive gut our frog-embryo has reached the gastrula stage, though it is clear that this cenogenetic amphibian gastrula is very different from the real palingenetic gastrula we have considered (Figures 1.30 to 1.36).

In the growth of this hooded gastrula we cannot sharply mark off the various stages which we distinguish successively in the bell-gastrula as morula and gastrula. Nevertheless, it is not difficult to reduce the whole cenogenetic or disturbed development of this amphigastrula to the true palingenetic formation of the archigastrula of the amphioxus.

(FIGURE 1.45. Blastula of the water-salamander (Triton). fh segmentation-cavity, dz yelk-cells, rz border-zone. (From Hertwig.)

FIGURE 1.46. Embryonic vesicle of triton (blastula), outer view, with the transverse fold of the primitive mouth (u). (From Hertwig.)

FIGURE 1.47. Sagittal section of a hooded-embryo (depula) of triton (blastula at the commencement of gastrulation). ak outer germinal layer, ik inner germinal layer, fh segmentation-cavity, ud primitive gut, u primitive mouth, dl and vl dorsal and ventral lips of the mouth, dz yelk-cells. (From Hertwig.))

This reduction becomes easier if, after considering the gastrulation of the tailless amphibia (frogs and toads), we glance for a moment at that of the tailed amphibia, the salamanders. In some of the latter, that have only recently been carefully studied, and that are phylogenetically older, the process is much simpler and clearer than is the case with the former and longer known. Our common water-salamander (Triton taeniatus) is a particularly good subject for observation. Its nutritive yelk is much smaller and its formative yelk less obscured with black pigment-cells than in the case of the frog; and its gastrulation has better retained the original palingenetic character. It was first described by Scott and Osborn (1879), and Oscar Hertwig especially made a careful study of it (1881), and rightly pointed out its great importance in helping us to understand the vertebrate development. Its globular blastula (Figure 1.45) consists of loosely-aggregated, yelk-filled entodermic cells or yelk-cells (dz) in the lower vegetal half; the upper, animal half encloses the hemispherical segmentation-cavity (fh), the curved roof of which is formed of two or three strata of small ectodermic cells.

At the point where the latter pa.s.s into the former (at the equator of the globular vesicle) we have the border zone (rz). The folding which leads to the formation of the gastrula takes place at a spot in this border zone, the primitive mouth (Figure 1.46 u).

Unequal segmentation takes place in some of the cyclostoma and in the oldest fishes in just the same way as in most of the amphibia. Among the cyclostoma ("round-mouthed") the familiar lampreys are particularly interesting. In respect of organisation and development they are half-way between the acrania (lancelet) and the lowest real fishes (Selachii); hence I divided the group of the cyclostoma in 1886 from the real fishes with which they were formerly a.s.sociated, and formed of them a special cla.s.s of vertebrates. The ovum-segmentation in our common river-lamprey (Petromyzon fluviatilis) was described by Max Schultze in 1856, and afterwards by Scott (1882) and Goette (1890).

Unequal total segmentation follows the same lines in the oldest fishes, the selachii and ganoids, which are directly descended from the cyclostoma. The primitive fishes (Selachii), which we must regard as the ancestral group of the true fishes, were generally considered, until a short time ago, to be discoblastic. It was not until the beginning of the twentieth century that Bashford Dean made the important discovery in j.a.pan that one of the oldest living fishes of the shark type (Cestracion j.a.ponicus) has the same total unequal segmentation as the amphiblastic plated fishes (ganoides).* (*

Bashford Dean, Holoblastic Cleavage in the Egg of a Shark, Cestracion j.a.ponicus Macleay. Annotationes zoologicae j.a.ponenses, volume 4 Tokio 1901.) This is particularly interesting in connection with our subject, because the few remaining survivors of this division, which was so numerous in paleozoic times, exhibit three different types of gastrulation. The oldest and most conservative forms of the modern ganoids are the scaly sturgeons (Sturiones), plated fishes of great evolutionary importance, the eggs of which are eaten as caviar; their cleavage is not essentially different from that of the lampreys and the amphibia. On the other hand, the most modern of the plated fishes, the beautifully scaled bony pike of the North American rivers (Lepidosteus), approaches the osseous fishes, and is discoblastic like them. A third genus (Amia) is midway between the sturgeons and the latter.

(FIGURE 1.48. Sagittal section of the gastrula of the water-salamander (Triton). (From Hertwig.) Letters as in Figure 1.47; except--p yelk-stopper, mk beginning of the middle germinal layer.)

The group of the lung-fishes (Dipneusta or Dipnoi) is closely connected with the older ganoids. In respect of their whole organisation they are midway between the gill-breathing fishes and the lung-breathing amphibia; they share with the former the shape of the body and limbs, and with the latter the form of the heart and lungs.

Of the older dipnoi (Paladipneusta) we have now only one specimen, the remarkable Ceratodus of East Australia; its amphiblastic gastrulation has been recently explained by Richard Semon (cf. Chapter 2.21). That of the two modern dipneusta, of which Protopterus is found in Africa and Lepidosiren in America, is not materially different. (Cf. Figure 1.51.)

(FIGURE 1.49. Ovum-segmentation of the lamprey (Petromyzon fluviatalis), in four successive stages. The small cells of the upper (animal) hemisphere divide much more quickly than the cells of the lower (vegetal) hemisphere.

FIGURE 1.50. Gastrulation of the lamprey (Petromyzon fluviatilis). A blastula, with wide embryonic cavity (blastocoel, bl), g incipient inv.a.g.i.n.ation. B depula, with advanced inv.a.g.i.n.ation, from the primitive mouth (g). C gastrula, with complete primitive gut: the embryonic cavity has almost disappeared in consequence of inv.a.g.i.n.ation.)

All these amphiblastic vertebrates, Petromyzon and Cestracion, Accipenser and Ceratodus, and also the salamanders and batrachia, belong to the old, conservative groups of our stem. Their unequal ovum-segmentation and gastrulation have many peculiarities in detail, but can always be reduced with comparative ease to the original cleavage and gastrulation of the lowest vertebrate, the amphioxus; and this is little removed, as we have seen, from the very simple archigastrula of the Sagitta and Monoxenia (see Figures 1.29 to 1.36).

All these and many other cla.s.ses of animals generally agree in the circ.u.mstance that in segmentation their ovum divides into a large number of cells by repeated cleavage. All such ova have been called, after Remak, "whole-cleaving" (holoblasta), because their division into cells is complete or total.

(FIGURE 1.51. Gastrulation of ceratodus (from Semon). A and C stage with four cells, B and D with sixteen cells. A and B are seen from above, C and D sideways. E stage with thirty-two cells; F blastula; G gastrula in longitudinal section. fh segmentation-cavity. gh primitive gut or gastric cavity.)

In a great many other cla.s.ses of animals this is not the case, as we find (in the vertebrate stem) among the birds, reptiles, and most of the fishes; among the insects and most of the spiders and crabs (of the articulates); and the cephalopods (of the molluscs). In all these animals the mature ovum, and the stem-cell that arises from it in fertilisation, consist of two different and separate parts, which we have called formative yelk and nutritive yelk. The formative yelk alone consists of living protoplasm, and is the active, evolutionary, and nucleated part of the ovum; this alone divides in segmentation, and produces the numerous cells which make up the embryo. On the other hand, the nutritive yelk is merely a pa.s.sive part of the contents of the ovum, a subordinate element which contains nutritive material (alb.u.min, fat, etc.), and so represents in a sense the provision-store of the developing embryo. The latter takes a quant.i.ty of food out of this store, and finally consumes it all. Hence the nutritive yelk is of great indirect importance in embryonic development, though it has no direct share in it. It either does not divide at all, or only later on, and does not generally consist of cells. It is sometimes large and sometimes small, but generally many times larger than the formative yelk; and hence it is that it was formerly thought the more important of the two. As the respective significance of these two parts of the ovum is often wrongly described, it must be borne in mind that the nutritive yelk is only a secondary addition to the primary cell, it is an inner enclosure, not an external appendage. All ova that have this independent nutritive yelk are called, after Remak, "partially-cleaving" (meroblasta). Their segmentation is incomplete or partial.

(FIGURE 1.52. Ovum of a deep-sea bony fish. b protoplasm of the stem-cell, k nucleus of same, d clear globule of alb.u.min, the nutritive yelk, f fat-globule of same, c outer membrane of the ovum, or ovolemma.)

There are many difficulties in the way of understanding this partial segmentation and the gastrula that arises from it. We have only recently succeeded, by means of comparative research, in overcoming these difficulties, and reducing this cenogenetic form of gastrulation to the original palingenetic type. This is comparatively easy in the small meroblastic ova which contain little nutritive yelk--for instance, in the marine ova of a bony fish, the development of which I observed in 1875 at Ajaccio in Corsica. I found them joined together in lumps of jelly, floating on the surface of the sea; and, as the little ovula were completely transparent, I could easily follow the development of the germ step by step. These ovula are glossy and colourless globules of little more than the 50th of an inch. Inside a structureless, thin, but firm membrane (ovolemma, Figure 1.52 c) we find a large, quite clear, and transparent globule of alb.u.min (d). At both poles of its axis this globule has a pit-like depression. In the pit at the upper, animal pole (which is turned downwards in the floating ovum) there is a bi-convex lens composed of protoplasm, and this encloses the nucleus (k); this is the formative yelk of the stem-cell, or the germinal disk (b). The small fat-globule (f) and the large alb.u.min-globule (d) together form the nutritive yelk. Only the formative yelk undergoes cleavage, the nutritive yelk not dividing at all at first.

The segmentation of the lens-shaped formative yelk (b) proceeds quite independently of the nutritive yelk, and in perfect geometrical order.