1.39 p): these are the important polar cells of the primitive mouth,
or "the primitive cells of the mesoderm." In consequence of these
considerable variations arising in the course of the gastrulation, the
primitive uni-axial form of the archigastrula in the amphioxus has
already become tri-axial, and thus the two-sidedness, or bilateral
symmetry, of the vertebrate body has already been determined. This has
been transmitted from the amphioxus to all the other modified
gastrula-forms of the vertebrate stem.

Apart from this bilateral structure, the gastrula of the amphioxus
resembles the typical archigastrula of the lower animals (Figures 1.30
to 1.36) in developing the two primary germinal layers from a single
layer of cells. This is clearly the oldest and original form of the
metazoic embryo. Although the animals I have mentioned belong to the
most diverse classes, they nevertheless agree with each other, and
many more animal forms, in having retained to the present day, by a
conservative heredity, this palingenetic form of gastrulation which
they have from their earliest common ancestors. But this is not the
case with the great majority of the animals. With these the original
embryonic process has been gradually more or less altered in the
course of millions of years by adaptation to new conditions of
development. Both the segmentation of the ovum and the subsequent
gastrulation have in this way been considerably changed. In fact,
these variations have become so great in the course of time that the
segmentation was not rightly understood in most animals, and the
gastrula was unrecognised. It was not until I had made an extensive
comparative study, lasting a considerable time (in the years 1866 to
1875), in animals of the most diverse classes, that I succeeded in
showing the same common typical process in these apparently very
different forms of gastrulation, and tracing them all to one original