As a rule, the cells which compose the two primary germinal layers show appreciable differences even in the gastrula stage. Generally (if not always) the cells of the skin-layer or ectoderm (Figs. 36 _c_ and 37 _e_) are the smaller, more numerous, and clearer; while the cells of the gut-layer, or entoderm (_i_), are larger, less numerous, and darker. The protoplasm of the ectodermic (outer) cells is clearer and firmer than the thicker and softer cell-matter of the entodermic (inner) cells; the latter are, as a rule, much richer in yelk-granules (albumen and fatty particles) than the former. Also the cells of the gut-layer have, as a rule, a stronger affinity for colouring matter, and take on a tinge in a solution of carmine, aniline, etc., more quickly and appreciably than the cells of the skin-layer. The nuclei of the entoderm-cells are usually roundish, while those of the ectoderm-cells are oval.
When the doubling-process is complete, very striking histological differences between the cells of the two layers are found (Fig. 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,_ Fig. 29), we find it in the lowest zoophyta (the gastrophysema, Fig. 30), and the simplest sponges (olynthus, Fig. 36); also in many of the medusæ and hydrapolyps, lower types of worms of various classes (brachiopod, arrow-worm, Fig. 31), tunicates (ascidia), many of the echinoderms (Fig. 32), lower articulates (Fig. 33), and molluscs (Fig. 34), and, finally, in a slightly modified form, in the lowest vertebrate (the amphioxus, Fig. 35).