differs in different classes of animals to such an extent that it is difficult to formulate any general description which will apply to all of them. Usually the many cleavage cells form a hollow sphere, the blastula (Fig. 10, K), and this in turn becomes a gastrula (Fig. 10, L, M), in which at first two, and later three, groups or layers of cells may be recognized; the outer layer, which is formed from cells nearest the animal pole of the egg, is the ectoderm; the inner layer, or entoderm, is formed from cells nearest the vegetative pole; a middle layer, or group of cells, the mesoderm, is formed from cleavage cells which in vertebrates lie between the animal and vegetative poles.
5. Organogeny.—By further differentiation of the cells of these layers and by dissimilar growth and folding of the layers themselves the various organs of the embryo begin to appear. From the ectoderm is formed the outer layer of the skin and the nervous system; from the entoderm arises the lining of the alimentary canal and its outgrowths; from the mesoderm comes, in whole or in part, the skeletal, muscular, vascular, excretory, and reproductive systems. In vertebrates the nervous system appears as a plate of rather large ectoderm cells (Fig. 12);
Fig. 12. Cross Section of Amphioxus Larvae in Successive Stages of Development. A, through a larva similar to 11A; B and C, of a larva similar to 11B; D, of a still older larva; ect, ectoderm; ent, entoderm; mes, mesoderm; ch, notochord; np, neural plate; gc, gastroccel; ac, alimentary canal; cæl, cœlom.
this plate rolls up at its sides to form a groove (Fig. 12) and then a tube (Fig. 12); and by enlargement of certain portions of this tube and by foldings and thickenings of its walls the brain and spinal cord are formed (Figs. 12, 15, C, D). The retina or sensory portion of the eye is formed as an outgrowth from the fore part of the brain (Fig. 15, D); the sensory portion of the ear comes from a cup-shaped depression of the superficial ectoderm which covers the hinder portion of the head (Fig. 15, E and F). The back bone begins to appear as a delicate cellu-