9. The primitive mouth, which at first lies below at the lower pole of the vertical axis, is forced, by the growth of the yelk, backwards and then upwards, towards the dorsal side of the embryo; the vertical axis of the primitive gut is thus gradually converted into horizontal.
10. The primitive mouth is closed sooner or later in all the vertebrates, and does not evolve into the permanent mouth-aperture; it rather corresponds to the “properistoma,” or region of the anus. From this important point the formation of the middle germinal layer proceeds, between the two primary layers.
The wide comparative studies of the scientists I have named have further shown that in the case of the discoblastic higher vertebrates (the three classes of amniotes) the primitive mouth of the embryonic disc, which was long looked for in vain, is found always, and is nothing else than the familiar “primitive groove.” Of this we shall see more as we proceed. Meantime we realise that gastrulation may be reduced to one and the same process in all the vertebrates. Moreover, the various forms it takes in the invertebrates can always be reduced to one of the four types of segmentation described above. In relation to the distinction between total and partial segmentation, the grouping of the various forms is as follows:—
I. Palingenetic (primitive) segmentation. 1. Equal segmentation (bell-gastrula). A. Total segmentation (without independent food-yelk). II. Cenogenetic segmentation (modified by adaptation). 2. Unequal segmentation (hooded gastrula). 3. Discoid segmentation (discoid gastrula). B. Partial segmentation (with independent food-yelk). 4. Superficial segmentation (spherical gastrula).
The lowest metazoa we know—namely, the lower zoophyta (sponges, simple polyps, etc.)—remain throughout life at a stage of development which differs little from the gastrula; their whole body consists of two layers of cells. This is a fact of extreme importance. We see that man, and also other vertebrates, pass quickly through a stage of development in which they consist of two layers, just as these lower zoophyta do throughout life. If we apply our biogenetic law to the matter, we at once reach this important conclusion. “Man and all the other animals which pass through the two-layer stage, or gastrula-form, in the course of their embryonic development, must descend from a primitive simple stem-form, the whole body of which consisted throughout life (as is the case with the lower zoophyta to-day) merely of two cell-strata or germinal layers.” We will call this primitive stem-form, with which we shall deal more fully later on, the _ gastræa_—that is to say, “primitive-gut animal.”