108 



TEXT-BOOK OF EMBRYOLOGY. 



vascular system in the egg-laying Amniotes has been noted. This is dependent 

 upon the fact that the embryo very early in its existence loses its nutritional con- 

 nection with its mother and is therefore dependent for its food upon the yolk 

 stored up within the egg. This condition obtains up through the lowest order 

 of Mammals, the Monotremes, which are egg-laying animals. The Marsupials 

 give birth to young of very immature development. In these two orders of 

 Mammals the foetal membranes present essentially the same condition as in 

 Birds and Reptiles. The chorion in Marsupials, however, lies in close ap- 

 position to the vascular uterine mucosa and perhaps provides for the passage of 



Chorion 



Muscularis 



FIG. 102. Vertical section through wall of uterus and chorion of a pig. Photograph. 



Note especially the close apposition of the chorionic and uterine epithelium (and compare with 



Fig. 103); note also the enlarged blood vessels in the uterine mucosa. 



nutrition from the mother to the embryo. In all higher Mammals, however, no 

 eggs are laid and the embryo early acquires an intimate nutritional relation to 

 its mother. This relation is maintained until the embryo has reached a com- 

 paratively advanced stage of development. As would be expected therefore, 

 there take place, coincidently with the change in nutritional relation between 

 mother and embryo, and dependent upon this changed relation, the already 

 noted decrease in, or entire loss of, yolk and at the same time the development of 

 a special organ of relation between embryo and uterus. This organ is devel- 

 oped mainly from the chorion which becomes highly specialized as compared 

 with the very simple chorion described in the chick. 



