The Evolution of Man, V.1. by Ernst Haeckel (most read book in the world .TXT) 📖

not a foreign external appendage of the embryo, but an outlying part of its primitive gut. The dorsal shield gradually separates from the germinative area and the yelk-sac, its edges growing downwards and folding together to form ventral plates. 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 passes 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 classes 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 mass 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 accumulation 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 XIII(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.

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 pronounced. 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 canalis 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 pass into each other in a very remarkable way. The point of transition is the remarkable neurenteric canal (Figure 1.124 cn). But the direct connection which is thus established does not last long; the two are soon definitely separated by a partition.

The enigmatic neurenteric canal 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 canal 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 canal (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 canals 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 canal 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 canal (Figure 1.126 ne) represents the arched connection between the dorsal medullary canal (mc) and the ventral gastric canal.

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

FIGURE 1.126. Longitudinal section of a frog-embryo. (From Goette.) m mouth, l liver, an anus, ne canalis 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 canal 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