When the doubling-process is complete, very striking histological
differences between the cells of the two layers are found (Figure
1.37). The tiny, light ectoderm-cells (e) are sharply distinguished
from the larger and darker entoderm-cells (i). Frequently this
differentiation of the cell-forms sets in at a very early stage,
during the segmentation-process, and is already very appreciable in
the blastula.

We have, up to the present, only considered that form of segmentation
and gastrulation which, for many and weighty reasons, we may regard as
the original, primordial, or palingenetic form. We might call it
"equal" or homogeneous segmentation, because the divided cells retain
a resemblance to each other at first (and often until the formation of
the blastoderm). We give the name of the "bell-gastrula," or
archigastrula, to the gastrula that succeeds it. In just the same form
as in the coral we considered (Monoxenia, Figure 1.29), we find it in
the lowest zoophyta (the gastrophysema, Figure 1.30), and the simplest
sponges (olynthus, Figure 1.36); also in many of the medusae and
hydrapolyps, lower types of worms of various classes (brachiopod,
arrow-worm, Figure 1.31), tunicates (ascidia), many of the echinoderms
(Figure 1.32), lower articulates (Figure 1.33), and molluscs (Figure
1.34), and, finally, in a slightly modified form, in the lowest
vertebrate (the amphioxus, Figure 1.35).

(FIGURE 1.37. Cells from the two primary germinal layers of the mammal
(from both layers of the blastoderm). i larger and darker cells of the
inner stratum, the vegetal layer or entoderm. e smaller and clearer
cells from the outer stratum, the animal layer or ectoderm.

FIGURE 1.38. Gastrulation of the amphioxus, from Hatschek (vertical