FACTS AND FACTORS OF DEVELOPMENT 33 



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,I> } 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 



