578 TEXT-BOOK OF EMBRYOLOGY. 



lium and also by some connective tissue of the chorion and of the uterine 

 mucosa (Fig. 490). Food materials for the embryo must, therefore, pass through 

 the connective tissue and the two epithelial layers in order to get from the 

 maternal to the foetal blood; and waste products from the embryo must also pass 

 through the same tissues to get from the fcetal to the maternal blood. When the 

 foetal membranes of the pig are expelled at birth, the rudimentary chorionic 

 villi simply withdraw from their sockets in the uterine mucosa and the chorion 

 is cast off, leaving the uterine mucosa intact. 



In other Mammals, the attachment of the chorion to the mucous membrane 

 of the uterus is restricted to certain definite, highly specialized areas. This 

 means that the villi which at first developed over the entire chorion, disappear 

 from the greater part of it. Those villi which remain are limited to a definite 

 area or areas and develop extensive arborizations. Moreover, they do not 



FIG. 491. Chorion of sheep, showing cotyledonary placenta. O. Schultze. 



simply fit into depressions in the uterine mucosa, but become much more 

 closely attached to it while the mucosa increases in thickness and in vascularity 

 over the villous areas. There are thus formed two distinct though intimately 

 associated parts of a structure which is known as the placenta the uterine part 

 being designated the maternal placenta or placenta uterina, the fcetal part the 

 placenta fcetalis. Such Mammals are grouped as Placentalia. In the sheep 

 and cow a number of placentae multiple placenta are normally present (Fig. 

 104). In the dog and cat the placenta takes the shape of a band or a zone 

 of specialized tissue encircling the germ vesicle. This is known as a zonular 

 placenta. In man a single discoidal area develops discoidal placenta. 



These different forms of placentae vary also in regard to the intimacy with 

 which maternal and fcetal parts are associated. Thus, for example, in the 

 multiple placentae of the cow and sheep, the fcetal placentae may be easily 



