516 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



As was first pointed out by Balfour, the types of 

 mammalian placentae fall primarily into two great 

 groups ; in one the yolk sac (or so-called umbilical 

 vesicle) takes a share in the formation of the placenta ; 

 this arrangement, which is probably the more primi- 

 tive, is retained by the Insectivora, Rodentia, and 

 Chiroptera. The other type, which is seen in all the 

 other forms, is characterised by the fact that the yolk 

 sac ceases to take any important share in the nourish- 

 ment of the foetus. 



Placentas, in which the foetal portion is derived 

 chiefly from the allantois, and not from the yolk sac, 

 fall again into two great divisions ; in the simpler 

 forms the outgrowths (villi) of the chorion merely 

 project into the pits which are developed in the uterus 

 of the mother, and may, at birth, be drawn out from 

 them without injuring the uterine vessels ; these are 

 the non-deciduate placentas. In the other the 

 connection between the walls of the villi and those of 

 the pits becomes so intimate that the vessels of the 

 uterus are torn when the foetus is born; such pla- 

 centaa are called deciduate. The former, again, 

 belong to one of two groups ; in the horse or the pig 

 the chorionic villi extend over nearly the whole of 

 the surface of the placenta, and we have a diffused 

 placenta ; in others, such as the sheep or the cow, 

 the villi are collected into definite tufts or branches, 

 which are distinctively known as cotyledons, and 

 the placenta is said to be cotyledonary. The spaces 

 between the cotyledons, of which the cow and the 

 goat have from sixty to one hundred, but the deer 

 only from eight to twelve, are left bare of villi. 



In the deciduata the placenta is ordinarily said to 

 be discoidal or zonary, but care must be taken in 

 the application of the former of these two terms, for 

 the placenta is disc-shaped in insectivores, rodents, and 

 bats, as well as in man and the apes ; it is convenient, 



