This phylogenetic explanation of gastrulation and cœlomation, 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. 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 cœlomula of the human embryo (Fig. 97), by transverse sections through its primitive mouth, that its two cœlom-pouches are developed in just the same way as in the rabbit (Fig. 96); moreover, the peculiar course of the gastrulation is just the same.
Fig.120. Embryonic vesicle of a seven-days-old rabbit with oval embryonic shield (ag). Fig. 120—Embryonic vesicle of a seven-days-old rabbit with oval embryonic shield (_ag_). _A_ seen from above, _B_ from the side. (From _Kölliker._) _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 XII. 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 (Fig. 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 (Fig. 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.