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,
Ruckert, 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.