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. IS np) ; this plate rolls up at 

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

 then a tube (Fig. 13 Z>) ; 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- 



