32 HEREDITY AND ENVIRONMENT 



depression of the superficial ectoderm which 

 covers the hinder portion of the head (Fig. 

 13, E and F). The back-bone begins to ap- 

 pear as a delicate cellular rod (Fig. 8, cln, 10 

 c), which then in higher vertebrates becomes 

 surrounded successively by a fibrous, a cartila- 

 ginous, and a bony sheath. And so one might 

 go on with a description of all the organs of 

 the body, each of which begins as a relatively 

 simple group or layer of cells, which gradu- 

 ally become more complicated by a process of 

 growth and differentiation, until these embry- 

 onic organs assume more and more the ma- 

 ture form. 



6. Oviparity and Viviparity. — This very 

 brief and general statement of the manner of 

 embryonic development applies to all verte- 

 brates, man included. There are many special 

 features of human development which are 

 treated at length in works on embryology, but 

 which need not detain us here since they do 

 not affect the general principles of develop- 

 ment already outlined. In one regard the de- 

 velopment of the human being or of any mam- 

 mal is apparently very different from that of 



