differ from one another. The differentiations of the cleavage 

 cells appear much earlier in some forms than in others, but in all 

 cases such differentiations appear during early or late cleavage 

 (Figs. 9-11). 



4. Embryogeny. From this stage onward the course of de- 

 velopment 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 (Figs. 9, n, //), and this in turn be- 

 comes a gastrula (Figs. 9, n, K), 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 upper pole of the 

 egg, is the ectoderm ; the inner layer, or endoderm, is formed from 

 cells nearest the lower pole; a middle layer, or group of cells, the 

 mesoderm, is formed from cleavage cells which in vertebrates lie 

 between the upper and lower poles (Fig. u, m). 



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 are formed the outer layer of the skin and the 

 whole nervous system ; from the endoderm arise the lining of the 

 alimentary canal and its outgrowths ; from the mesoderm come, 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. n, n) ; this plate 

 rolls up at its sides to form a groove and then a tube ; and by en- 

 largement of certain portions of this tube and by foldings and 

 thickenings of its walls the brain and spinal cord are formed 

 (Fig. n, K, L; 13, C, D). The retina or sensory portion of the 

 eye is formed as an outgrowth from the fore part of the brain 

 (Fig. 13, 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. 13, E and F). The back-bone 

 begins to appear as a delicate cellular rod (Fig. n, c), which then 

 in higher vertebrates becomes surrounded successively by a 



