28 HEREDITY AND ENVIRONMENT 



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 (Fig. 13 np) ; this plate rolls up at 

 its sides to form a groove (Fig. 13 C) and 

 then a tube (Fig. 13 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. 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 depres- 

 sion 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 deli- 



