30 HEREDITY AND ENVIRONMENT 



ferentiation, until these embryonic organs as- 

 sume more and more the mature 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 

 a bird or frog or fish, viz., in the fact that in 

 the former the embryonic development takes 

 place within the body of the mother whereas 

 in the latter the eggs are laid before or soon 

 after fertilization. In man, after the cleavage 

 of the egg, a hollow vesicle is formed, which 

 becomes attached to the uterine walls by 

 means of processes or villi which grow out 

 from it (Fig. 14, D, E, F) while only a small 

 portion of the vesicle becomes transformed 

 into the embryo. There is thus established a 

 connection between the embryo and the uterine 



