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 section through the axis of the ovum). A, B, C three stages in the formation of the blastula; D, E curving of the blastula; F complete gastrula. h segmentation-cavity. g primitive gut-cavity.))