When the animal ovum (egg-cell) has been fertilised, it divides and
subdivides until we have a cluster of cohering cells, externally not
unlike a raspberry or mulberry. This is the morula (= mulberry) stage.
The cluster becomes hollow, or filled with fluid in the centre, all
the cells rising to the surface. This is the blastula (hollow ball)
stage. One half of the cluster then bends or folds in upon the other,
as one might do with a thin indiarubber ball, and we get a vase-shaped
body with hollow interior (the first stomach, or "primitive gut"), an
open mouth (the first or "primitive mouth"), and a wall composed of
two layers of cells (two "germinal layers"). This is the gastrula
(stomach) stage, and the process of its formation is called
gastrulation. A glance at the illustration (Figure 1.29) will make
this perfectly clear.

So much for the embryonic process in itself. The application to
evolution has been a long and laborious task. Briefly, it was
necessary to show that ALL the multicellular animals passed through
these three stages, so that our biogenetic law would enable us to
recognise them as reminiscences of ancestral forms. This is the work
of Chapters 1.8 and 1.9. The difficulty can be realised in this way:
As we reach the higher animals the ovum has to take up a large
quantity of yelk, on which it may feed in developing. Think of the
bird's "egg." The effect of this was to flatten the germ (the morula
and blastula) from the first, and so give, at first sight, a totally
different complexion to what it has in the lowest animals. When we
pass the reptile and bird stage, the large yelk almost disappears (the
germ now being supplied with blood by the mother), but the germ has
been permanently altered in shape, and there are now a number of new
embryonic processes (membranes, blood-vessel connections, etc.). Thus
it was no light task to trace the identity of this process of