FACTS AND FACTORS OF DEVELOPMENT 31 



of cells, the mesoderm, is formed from cleav- 

 age cells which in vertebrates lie between the 

 upper and lower poles (Fig. 10, 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 ap- 

 pear. From the ectoderm is formed the outer 

 layer of the skin and the nervous system ; from 

 the endoderm arise the lining of the aliment- 

 ary canal and its outgrowths; from the meso- 

 derm come, in whole or in part, the skeletal, 

 muscular, vascular, excretory, and reproduc- 

 tive systems. In vertebrates the nervous sys- 

 tem appears as a plate of rather large ecto- 

 derm cells (Figs. 8, 10 np) ; this plate rolls up 

 at its sides to form a groove (Fig. 8 C) and 

 then a tube (Fig. 8 D) ; and by enlarge- 

 ment of certain portions of this tube and by 

 foldings and thickenings of its walls the brain 

 and spinal cord are formed (Fig. 10 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 



