Man is like all the other higher animals, especially the apes, in regard to these earliest and most important processes. As the human embryo does not essentially differ, even at a much later stage of development--when we already perceive the cerebral vesicles, the eyes, ears, gill-arches, etc.--from the similar forms of the other higher mammals, we may confidently assume that they agree in the earliest embryonic processes, segmentation and the formation of germinal layers. This has not yet, it is true, been established by observation. We have never yet had occasion to dissect a woman immediately after impregnation and examine the stem-cell or the segmentation-cells in her oviduct. However, as the earliest human embryos we have examined, and the later and more developed forms, agree with those of the rabbit, dog, and other higher mammals, no reasonable man will doubt but that the segmentation and formation of layers are the same in both cases. But the special form of segmentation and layer formation which we find in the mammal is by no means the original, simple, palingenetic form. It has been much modified and cenogenetically altered by a very complex adaptation to embryonic conditions. We cannot, therefore, 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.)