understand it altogether in itself. In order to do this, we have to
make a COMPARATIVE study of segmentation and layer-formation in the
animal world; and we have especially to seek the original,
PALINGENETIC form from which the modified CENOGENETIC (see Chapter
1.1) form has gradually been developed.

This original unaltered form of segmentation and layer-formation is
found to-day in only one case in the vertebrate-stem to which man
belongs--the lowest and oldest member of the stem, the wonderful
lancelet or amphioxus (cf. Chapters 2.16 and 2.17). But we find a
precisely similar palingenetic form of embryonic development in the
case of many of the invertebrate animals, as, for instance, the
remarkable ascidia, the pond-snail (Limnaeus), and arrow-worm
(Sagitta), and many of the echinoderms and cnidaria, such as the
common star-fish and sea-urchin, many of the medusae and corals, and
the simpler sponges (Olynthus). We may take as an illustration the
palingenetic segmentation and germinal layer-formation in an
eight-fold insular coral, which I discovered in the Red Sea, and
described as Monoxenia Darwinii.

(FIGURE 1.29. Gastrulation of a coral (Monoxenia Darwinii). A, B,
stem-cell (cytula) or impregnated ovum. In Figure A (immediately after
impregnation) the nucleus is invisible. In Figure B (a little later)
it is quite clear. C two segmentation-cells. D four
segmentation-cells. E mulberry-formation (morula). F blastosphere
(blastula). G blastula (transverse section). H depula, or hollowed
blastula (transverse section). I gastrula (longitudinal section). K
gastrula, or cup-sphere, external appearance.)

The impregnated ovum of this coral (Figure 1.29 A, B) first splits