unicellular protists) the segmentation of the ovum produces either a
pure, primitive, palingenetic gastrula (Figure 1.29 I, K) or an
equally instructive cenogenetic form, which has been developed in time
from the first, and can be directly reduced to it. It is certainly a
fact of the greatest interest and instructiveness that animals of the
most different stems--vertebrates and tunicates, molluscs and
articulates, echinoderms and annelids, cnidaria and sponges--proceed
from one and the same embryonic form. In illustration I give a few
pure gastrula forms from various groups of animals (Figures 1.30 to
1.35, explanation given below each).

(FIGURES 1.30 TO 1.35. In each figure d is the primitive-gut cavity, o
primitive mouth, s segmentation-cavity, i entoderm (gut-layer), e
ectoderm (skin layer).

FIGURE 1.30. (A) Gastrula of a very simple primitive-gut animal or
gastraead (gastrophysema). (Haeckel.)

FIGURE 1.31. (B) Gastrula of a worm (Sagitta). (From Kowalevsky.)

FIGURE 1.32. (C) Gastrula of an echinoderm (star-fish, Uraster), not
completely folded in (depula). (From Alexander Agassiz.)

FIGURE 1.33. (D) Gastrula of an arthropod (primitive crab, Nauplius)
(as 32).

FIGURE 1.34. (E) Gastrula of a mollusc (pond-snail, Linnaeus). (From
Karl Rabl.)

FIGURE 1.35. (F) Gastrula of a vertebrate (lancelet, Amphioxus). (From