26 Heredity and Environment 



fibrous, a cartilaginous, 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 

 gradually become more complicated by a process of growth and 

 differentiation, until these embryonic organs assume 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 

 vertebrates, man included. There are many special features of 

 human development which are treated at length in works on em- 

 bryology, but which need not detain us here since they do not 

 affect the general principles of development already outlined. 

 In one regard the development of the human being or of almost 

 any mammal 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. 12, 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 walls through which nutriment is absorbed by the embryo. 

 And yet this difference is not a fundamental one for in different 

 animals there are all stages of transition between these two modes 

 of development. While in most fishes, amphibians and reptiles the 

 eggs are laid at the beginning of development and are free and 

 independent during the whole course of ontogeny, there are certain 

 species in each of these classes in which the development takes 

 place within the body of the mother. Even in birds a portion of 

 the development takes place within the body of the female before 

 the eggs are laid, and there are mammals (monotremes) which 

 lay eggs, while in others (marsupials) the young are born in a very 

 imperfect condition. 



Mother always Distinct from Child. — These facts indicate that 



