The characteristic embryonic form that the developing mammal now exhibits has up to the present usually been called the "blastula"

(Bischoff), "sac-shaped embryo" (Baer), "vesicular embryo" (vesicula blastodermica, or, briefly, blastosphaera). The wall of the hollow vesicle, which consists of a single layer of cells, was called the "blastoderm," and was supposed to be equivalent to the cell-layer of the same name that forms the wall of the real blastula of the amphioxus and many of the invertebrates (such as Monoxenia, Figure 1.29 F, G). Formerly this real blastula was generally believed to be equivalent to the embryonic vesicle of the mammal. However, this is by no means the case. What is called the "blastula" of the mammal and the real blastula of the amphioxus and many of the invertebrates are totally different embryonic structures. The latter (blastula) is palingenetic, and precedes the formation of the gastrula. The former (blastodermic vesicle) is cenogenetic, and follows gastrulation. The globular wall of the blastula is a real blastoderm, and consists of h.o.m.ogeneous (blastodermic) cells; it is not yet differentiated into the two primary germinal layers. But the globular wall of the mammal vesicle is the differentiated ectoderm, and at one point in it we find a circular disk of quite different cells--the entoderm. The round cavity, filled with fluid, inside the real blastula is the segmentation-cavity. But the similar cavity within the mammal vesicle is the yelk-sac cavity, which is connected with the incipient gut-cavity. This primitive gut-cavity pa.s.ses directly into the segmentation-cavity in the mammals, in consequence of the peculiar cenogenetic changes in their gastrulation, which we have considered previously (Chapter 1.9). For these reasons it is very necessary to recognise the secondary embryonic vesicle in the mammal (gastrocystis or blastocystis) as a characteristic structure peculiar to this cla.s.s, and distinguish it carefully from the primary blastula of the amphioxus and the invertebrates.

(FIGURE 1.110. Ovum of a rabbit from the uterus, one sixth of an inch in diameter. The embryonic vesicle (b) has withdrawn a little from the smooth ovolemma (a). In the middle of the ovolemma we see the round germinal disk (blastodiscus, c), at the edge of which (at d) the inner layer of the embryonic vesicle is already beginning to expand.

(Figures 1.110 to 1.114 from Bischoff.)

FIGURE 1.111. The same ovum, seen in profile. Letters as in Figure 1.110.

FIGURE 1.112. Ovum of a rabbit from the uterus, one-fourth of an inch in diameter. The blastoderm is already for the most part two-layered (b). The ovolemma, or outer envelope, is tufted (a).

FIGURE 1.113. The same ovum, seen in profile. Letters as in Figure 1.112.

FIGURE 1.114. Ovum of a rabbit from the uterus, one-third of an inch in diameter. The embryonic vesicle is now nearly everywhere two-layered (k) only remaining one-layered below (at d).

FIGURE 1.115. Round germinative area of the rabbit, divided into the central light area (area pellucida) and the peripheral dark area (area opaca). The light area seems darker on account of the dark ground appearing through it.)

The small, circular, whitish, and opaque spot which the gastric disk (Figure 1.106) forms at a certain part of the surface of the clear and transparent embryonic vesicle has long been known to science, and compared to the germinal disk of the birds and reptiles. Sometimes it has been called the germinal disk, sometimes the germinal spot, and usually the germinative area. From the area the further development of the embryo proceeds. However, the larger part of the embryonic vesicle of the mammal is not directly used for building up the later body, but for the construction of the temporary umbilical vesicle. The embryo separates from this in proportion as it grows at its expense; the two are only connected by the yelk-duct (the stalk of the yelk-sac), and this maintains the direct communication between the cavity of the umbilical vesicle and the forming visceral cavity (Figure 1.105).

The germinative area or gastric disk of the animal consists at first (like the germinal disk of birds and reptiles) merely of the two primary germinal layers, the ectoderm and entoderm. But soon there appears in the middle of the circular disk between the two a third stratum of cells, the rudiment of the middle layer or fibrous layer (mesoderm). This middle germinal layer consists from the first, as we have seen in Chapter 1.10, of two separate epithelial plates, the two layers of the coelom-pouches (parietal and visceral). However, in all the amniotes (on account of the large formation of yelk) these thin middle plates are so firmly pressed together that they seem to represent a single layer. It is thus peculiar to the amniotes that the middle of the germinative area is composed of four germinal layers, the two limiting (or primary) layers and the middle layers between them (Figures 1.96 and 1.97). These four secondary germinal layers can be clearly distinguished as soon as what is called the sickle-groove (or "embryonic sickle") is seen at the hind border of the germinative area. At the borders, however, the germinative area of the mammal only consists of two layers. The rest of the wall of the embryonic vesicle consists at first (but only for a short time in most of the mammals) of a single layer, the outer germinal layer.

(FIGURE 1.116. Oval area, with the opaque whitish border of the dark area without.)

From this stage, however, the whole wall of the embryonic vesicle becomes two-layered. The middle of the germinative area is much thickened by the growth of the cells of the middle layers, and the inner layer expands at the same time, and increases at the border of the disk all round. Lying close on the outer layer throughout, it grows over its inner surface at all points, covers first the upper and then the lower hemisphere, and at last closes in the middle of the inner layer (Figures 1.110 to 1.114). The wall of the embryonic vesicle now consists throughout of two layers of cells, the ectoderm without and the entoderm within. It is only in the centre of the circular area, which becomes thicker and thicker through the growth of the middle layers, that it is made up of all four layers. At the same time, small structureless tufts or warts are deposited on the surface of the outer ovolemma or prochorion, which has been raised above the embryonic vesicle (Figures 1.112 to 1.114 a).

(FIGURE 1.117. Oval germinal disk of the rabbit, magnified about ten times. As the delicate, half-transparent disk lies on a black ground, the pellucid area looks like a dark ring, and the opaque area (lying outside it) like a white ring. The oval shield in the centre also looks whitish, and in its axis we see the dark medullary groove. (From Bischoff.))

We may now disregard both the outer ovolemma and the greater part of the vesicle, and concentrate our attention on the germinative area and the four-layered embryonic disk. It is here alone that we find the important changes which lead to the differentiation of the first organs. It is immaterial whether we examine the germinative area of the mammal (the rabbit, for instance) or the germinal disk of a bird or a reptile (such as a lizard or tortoise). The embryonic processes we are now going to consider are essentially the same in all members of the three higher cla.s.ses of vertebrates which we call the amniotes.

Man is found to agree in this respect with the rabbit, dog, ox, etc.; and in all these animals the germinative area undergoes essentially the same changes as in the birds and reptiles. They are most frequently and accurately studied in the chick, because we can have incubated hens' eggs in any quant.i.ty at any stage of development.

Moreover, the round germinal disk of the chick pa.s.ses immediately after the beginning of incubation (within a few hours) from the two-layered to the four-layered stage, the two-layered mesoderm developing from the median primitive groove between the ectoderm and entoderm (Figures 1.82 to 1.95).

The first change in the round germinal disk of the chick is that the cells at its edges multiply more briskly, and form darker nuclei in their protoplasm. This gives rise to a dark ring, more or less sharply set off from the lighter centre of the germinal disk (Figure 1.115).

From this point the latter takes the name of the "light area" (area pellucida), and the darker ring is called the "dark area" (area opaca). (In a strong light, as in Figures 1.115 to 1.117, the light area seems dark, because the dark ground is seen through it; and the dark area seems whiter). The circular shape of the area now changes into elliptic, and then immediately into oval (Figures 1.116 and 1.117). One end seems to be broader and blunter, the other narrower and more pointed; the former corresponds to the anterior and the latter to the posterior section of the subsequent body. At the same time, we can already trace the characteristic bilateral form of the body, the ant.i.thesis of right and left, before and behind. This will be made clearer by the "primitive streak," which appears at the posterior end.

(FIGURE 1.118. Pear-shaped germinal shield of the rabbit (eight days old), magnified twenty times. rf medullary groove. pr primitive groove (primitive mouth). (From Kolliker.)

FIGURE 1.119. Median longitudinal section of the gastrula of four vertebrates. (From Rabl.) A discogastrula of a shark (Pristiurus). B amphigastrula of a sturgeon (Accipenser). C amphigastrula of an amphibium (Triton). D epigastrula of an amniote (diagram). a ventral, b dorsal lip of the primitive mouth.)

At an early stage an opaque spot is seen in the middle of the clear germinative area, and this also pa.s.ses from a circular to an oval shape. At first this shield-shaped marking is very delicate and barely perceptible; but it soon becomes clearer, and now stands out as an oval shield, surrounded by two rings or areas (Figure 1.117). The inner and brighter ring is the remainder of the pellucid area, and the dark outer ring the remainder of the opaque area; the opaque shield-like spot itself is the first rudiment of the dorsal part of the embryo. We give it briefly the name of embryonic shield or dorsal shield. In most works this embryonic shield is described as "the first rudiment or trace of the embryo," or "primitive embryo." But this is wrong, though it rests on the authority of Baer and Bischoff. As a matter of fact, we already have the embryo in the stem-cell, the gastrula, and all the subsequent stages. The embryonic shield is simply the first rudiment of the dorsal part, which is the earliest to develop. As the older names of "embryonic rudiment" and "germinative area" are used in many different senses--and this has led to a fatal confusion in embryonic literature--we must explain very clearly the real significance of these important embryonic parts of the amniote.

It will be useful to do so in a series of formal principles:--

1. The so-called "first trace of the embryo" in the amniotes, or the embryonic shield, in the centre of the pellucid area, consists merely of an early differentiation and formation of the middle dorsal parts.

2. Hence the best name for it is "the dorsal shield," as I proposed long ago.

3. The germinative area, in which the first embryonic blood-vessels appear at an early stage, is not opposed as an external area to the "embryo proper," but is a part of it.

4. In the same way, the yelk-sac or the umbilical vesicle is not a foreign external appendage of the embryo, but an outlying part of its primitive gut.

5. The dorsal shield gradually separates from the germinative area and the yelk-sac, its edges growing downwards and folding together to form ventral plates.

6. The yelk-sac and vessels of the germinative area, which soon spread over its whole surface, are, therefore, real embryonic organs, or temporary parts of the embryo, and have a transitory importance in connection with the nutrition of the growing later body; the latter may be called the "permanent body" in contrast to them.

The relation of these cenogenetic features of the amniotes to the palingenetic structures of the older non-amniotic vertebrates may be expressed in the following theses: The original gastrula, which completely pa.s.ses into the embryonic body in the acrania, cyclostoma, and amphibia, is early divided into two parts in the amniotes--the embryonic shield, which represents the dorsal outline of the permanent body; and the temporary embryonic organs of the germinative area and its blood-vessels, which soon grow over the whole of the yelk-sac. The differences which we find in the various cla.s.ses of the vertebrate stem in these important particulars can only be fully understood when we bear in mind their phylogenetic relations on the one hand, and, on the other, the cenogenetic modifications of structure that have been brought about by changes in the rearing of the young and the variation in the ma.s.s of the food-yelk.

We have already described in Chapter 1.9 the changes which this increase and decrease of the nutritive yelk causes in the form of the gastrula, and especially in the situation and shape of the primitive mouth. The primitive mouth or prostoma is originally a simple round aperture at the lower pole of the long axis; its dorsal lip is above and ventral lip below. In the amphioxus this primitive mouth is a little eccentric, or shifted to the dorsal side (Figure 1.39). The aperture increases with the growth of the food-yelk in the cyclostoma and ganoids; in the sturgeon it lies almost on the equator of the round ovum, the ventral lip (a) in front and the dorsal lip (b) behind (Figure 1.119 b). In the wide-mouthed, circular discoid gastrula of the selachii or primitive fishes, which spreads quite flat on the large food-yelk, the anterior semi-circle of the border of the disk is the ventral, and the posterior semicircle the dorsal lip (Figure 1.119 A). The amphiblastic amphibia are directly connected with their earlier fish-ancestors, the dipneusts and ganoids, and further the oldest selachii (Cestracion); they have retained their total unequal segmentation, and their small primitive mouth (Figure 1.119 C, ab), blocked up by the yelk-stopper, lies at the limit of the dorsal and ventral surface of the embryo (at the lower pole of its equatorial axis), and there again has an upper dorsal and a lower ventral lip (a, b). The formation of a large food-yelk followed again in the stem-forms of the amniotes, the protamniotes or proreptilia, descended from the amphibia (Figure 1.119 D). But here the acc.u.mulation of the food-yelk took place only in the ventral wall of the primitive-gut, so that the narrow primitive mouth lying behind was forced upwards, and came to lie on the back of the discoid "epigastrula" in the shape of the "primitive groove"; thus (in contrast to the case of the selachii, Figure 1.119 A) the dorsal lip (b) had to be in front, and the ventral lip (a) behind (Figure 1.119 D). This feature was transmitted to all the amniotes, whether they retained the large food-yelk (reptiles, birds, and monotremes), or lost it by atrophy (the viviparous mammals).

This phylogenetic explanation of gastrulation and coelomation, and the comparative study of them in the various vertebrates, throw a clear and full light on many ontogenetic phenomena, as to which the most obscure and confused opinions were prevalent thirty years ago. In this we see especially the high scientific value of the biogenetic law and the careful separation of palingenetic from cenogenetic processes. To the opponents of this law the real explanation of these remarkable phenomena is impossible. Here, and in every other part of embryology, the true key to the solution lies in phylogeny.

CHAPTER 1.13. DORSAL BODY AND VENTRAL BODY.

The earliest stages of the human embryo are, for the reasons already given, either quite unknown or only imperfectly known to us. But as the subsequent embryonic forms in man behave and develop just as they do in all the other mammals, there cannot be the slightest doubt that the preceding stages also are similar. We have been able to see in the coelomula of the human embryo (Figure 1.97), by transverse sections through its primitive mouth, that its two coelom-pouches are developed in just the same way as in the rabbit (Figure 1.96); moreover, the peculiar course of the gastrulation is just the same.

(FIGURE 1.120. Embryonic vesicle of a seven-days-old rabbit with oval embryonic shield (ag).

A seen from above, B from the side. (From Kolliker.) ag dorsal shield or embryonic spot. In B the upper half of the vesicle is made up of the two primary germinal layers, the lower (up to ge) only from the outer layer.)

The germinative area forms in the human embryo in the same way as in the other mammals, and in the middle part of this we have the embryonic shield, the purport of which we considered in Chapter 1.12.

The next changes in the embryonic disk, or the "embryonic spot," take place in corresponding fashion. These are the changes we are now going to consider more closely.

The chief part of the oval embryonic shield is at first the narrow hinder end; it is in the middle line of this that the primitive streak appears (Figure 1.121 ps). The narrow longitudinal groove in it--the so-called "primitive groove"--is, as we have seen, the primitive mouth of the gastrula. In the gastrula-embryos of the mammals, which are much modified cenogenetically, this cleft-shaped prostoma is lengthened so much that it soon traverses the whole of the hinder half of the dorsal shield; as we find in a rabbit embryo of six to eight days (Figure 1.122 pr). The two swollen parallel borders that limit this median furrow are the side lips of the primitive mouth, right and left. In this way the bilateral-symmetrical type of the vertebrate becomes p.r.o.nounced. The subsequent head of the amniote is developed from the broader and rounder fore-half of the dorsal shield.

In this fore-half of the dorsal shield a median furrow quickly makes its appearance (Figure 1.123 rf). This is the broader dorsal furrow or medullary groove, the first beginning of the central nervous system.

The two parallel dorsal or medullary swellings that enclose it grow together over it afterwards, and form the medullary tube. As is seen in transverse sections, it is formed only of the outer germinal layer (Figures 1.95 and 1.136). The lips of the primitive mouth, however, lie, as we know, at the important point where the outer layer bends over the inner, and from which the two coelom pouches grow between the primary germinal layers.

(FIGURE 1.121. Oval embryonic shield of the rabbit (A of six days eighteen hours, B of eight days). (From Kolliker.) ps primitive streak, pr primitive groove, arg area germinalis, sw sickle-shaped germinal growth.

FIGURE 1.122. Dorsal shield (ag) and germinative area of a rabbit-embryo of eight days. (From Kolliker.) pr primitive groove, rf dorsal furrow.

FIGURE 1.123. Embryonic shield of a rabbit of eight days. (From Van Beneden.) pr primitive groove, cn ca.n.a.lis neurentericus, nk nodus neurentericus (or "Hensen's ganglion"), kf head-process (chorda).

FIGURE 1.124. Longitudinal section of the coelomula of amphioxus (from the left). i entoderm, d primitive gut, cn medullary duct, n nerve tube, m mesoderm, s first primitive segment, c coelom-pouches. (From Hatschek.))

Thus the median primitive furrow (pr) in the hind-half and the median medullary furrow (rf) in the fore-half of the oval shield are totally different structures, although the latter seems to a superficial observer to be merely the forward continuation of the former. Hence they were formerly always confused. This error was the more pardonable as immediately afterwards the two grooves do actually pa.s.s into each other in a very remarkable way. The point of transition is the remarkable neurenteric ca.n.a.l (Figure 1.124 cn). But the direct connection which is thus established does not last long; the two are soon definitely separated by a part.i.tion.

The enigmatic neurenteric ca.n.a.l is a very old embryonic organ, and of great phylogenetic interest, because it arises in the same way in all the chordonia (both tunicates and vertebrates). In every case it touches or embraces like an arch the posterior end of the chorda, which has been developed here in front out of the middle line of the primitive gut (between the two coelom-folds of the sickle groove) ("head-process," Figure 1.123 kf). These very ancient and strictly hereditary structures, which have no physiological significance to-day, deserve (as "rudimentary organs") our closest attention. The tenacity with which the useless neurenteric ca.n.a.l has been transmitted down to man through the whole series of vertebrates is of equal interest for the theory of descent in general, and the phylogeny of the chordonia in particular.

The connection which the neurenteric ca.n.a.l (Figure 1.123 cn) establishes between the dorsal nerve-tube (n) and the ventral gut-tube (d) is seen very plainly in the amphioxus in a longitudinal section of the coelomula, as soon as the primitive mouth is completely closed at its hinder end. The medullary tube has still at this stage an opening at the forward end, the neuroporus (Figure 1.83 np). This opening also is afterwards closed. There are then two completely closed ca.n.a.ls over each other--the medullary tube above and the gastric tube below, the two being separated by the chorda. The same features as in the acrania are exhibited by the related tunicates, the ascidiae.

Again, we find the neurenteric ca.n.a.l in just the same form and situation in the amphibia. A longitudinal section of a young tadpole (Figure 1.125) shows how we may penetrate from the still open primitive mouth (x) either into the wide primitive gut-cavity (al) or the narrow overlying nerve-tube. A little later, when the primitive mouth is closed, the narrow neurenteric ca.n.a.l (Figure 1.126 ne) represents the arched connection between the dorsal medullary ca.n.a.l (mc) and the ventral gastric ca.n.a.l.

(FIGURE 1.125. Longitudinal section of the chordula of a frog. (From Balfour.) nc nerve-tube, x ca.n.a.lis neurentericus, al alimentary ca.n.a.l, yk yelk-cells, m mesoderm.

FIGURE 1.126. Longitudinal section of a frog-embryo. (From Goette.) m mouth, l liver, an a.n.u.s, ne ca.n.a.lis neurentericus, mc medullary-tube, pn pineal body (epiphysis), ch chorda.

FIGURES 1.127 AND 1.128. Dorsal shield of the chick. (From Balfour.) The medullary furrow (me), which is not yet visible in Figure 1.130, encloses with its hinder end the fore end of the primitive groove (pr) in Figure 1.131.)

In the amniotes this original curved form of the neurenteric ca.n.a.l cannot be found at first, because here the primitive mouth travels completely over to the dorsal surface of the gastrula, and is converted into the longitudinal furrow we call the primitive groove.

Hence the primitive groove (Figure 1.128 pr), examined from above, appears to be the straight continuation of the fore-lying and younger medullary furrow (me). The divergent hind legs of the latter embrace the anterior end of the former. Afterwards we have the complete closing of the primitive mouth, the dorsal swellings joining to form the medullary tube and growing over it. The neurenteric ca.n.a.l then leads directly, in the shape of a narrow arch-shaped tube (Figure 1.129 ne), from the medullary tube (sp) to the gastric tube (pag).

Directly in front of it is the latter end of the chorda (cli).