32 HEREDITY AND ENVIRONMENT 



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 (mono- 

 tremes) which lay eggs, while in others (mar- 

 supials) the young are born in a very imperfect 

 condition. These facts indicate that there is no 

 fundamental difference between oviparity and 

 viviparity. In the latter the union between the 

 embryo and the mother is a nutritive but not a 

 protoplasmic one. Blood plasma passes from 

 one to the other by a process of soakage, and 

 the only material influences which can affect 

 the developing embryo are such as may be 

 conveyed through the blood plasma and are 



