The gastrulation of the discoblastic ova, which chiefly concerns us, offers serious difficulties to microscopic investigation and philosophic consideration. These, however, have been mastered by the comparative embryological research which has been conducted by a number of distinguished observers during the last few decades—especially the brothers Hertwig, Rabl, Kupffer, Selenka, Rückert, Goette, Rauber, etc. These thorough and careful studies, aided by the most perfect modern improvements in technical method (in tinting and dissection), have given a very welcome support to the views which I put forward in my work, _On the Gastrula and the Segmentation of the Animal Ovum_ [not translated], in 1875. As it is very important to understand these views and their phylogenetic foundation clearly, not only as regards evolution in general, but particularly in connection with the genesis of man, I will give here a brief statement of them as far as they concern the vertebrate-stem:—
1. All the vertebrates, including man, are phylogenetically (or genealogically) related—that is, are members of one single natural stem.
2. Consequently, the embryonic features in their individual development must also have a genetic connection.
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.