3. As the gastrulation of the amphioxus shows the original palingenetic form in its simplest features, that of the other vertebrates must have been derived from it.
4. The cenogenetic modifications of the latter are more appreciable the more food-yelk is stored up in the ovum.
5. Although the mass of the food-yelk may be very large in the ova of the discoblastic vertebrates, nevertheless in every case a blastula is developed from the morula, as in the holoblastic ova.
6. Also, in every case, the gastrula develops from the blastula by curving or invagination.
7. The cavity which is produced in the fœtus by this curving is, in each case, the primitive gut (progaster), and its opening the primitive mouth (prostoma).
8. The food-yelk, whether large or small, is always stored in the ventral wall of the primitive gut; the cells (called “merocytes”) which may be formed in it subsequently (by “after-segmentation”) also belong to the inner germinal layer, like the cells which immediately enclose the primitive gut-cavity.
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 (withoutindependent food-yelk). |
| II. Cenogenetic segmentation (modified by adaptation). | 2. Unequal segmentation (hoodedgastrula). | |
| 3. Discoid segmentation (discoidgastrula). | B. Partialsegmentation (with independent food-yelk). | |
| 4. Superficial segmentation (spherical gastrula). |