THE EMBRYONIC MEMBRANES 253 



to note that these mammals are more closely related to one 

 another than to other mammals. This type of placenta 

 occupies a disc-shaped region of the trophoblast, whence its 

 name discoidal. 



At birth, the allantois and placenta are nipped off from the 

 embryo ; and the placenta separates from the uterus, and is 

 expelled as the " after-birth." In the carnivores (type iii) 

 this entails a certain amount of loss of maternal tissue ; in 

 the others the mother only loses blood. In Perameles, on the 

 other hand, the placenta is absorbed by the uterus. 



It must be remembered that as well as being an organ of 

 exchange between mother and embryo, the placenta functions 

 during early stages of development as a regulator of meta- 

 bolism of substances such as glycogen. Later on, this function 

 is taken on by the liver of the embryo. 



The Amnion. — The amnion is found only in reptiles, birds, 

 and mammals. All these animals differ from the fish and 

 amphibia in that the eggs are laid on dry land and not in water. 

 The amnion is formed by folds of the extra-embryonic ectoderm 

 and underlying mesoderm which rise up on all sides of the 

 embryo and meet above it. The inner layer so formed en- 

 closing the amniotic cavity is the amnion proper ; the outer 

 layer is the chorion. In the reptiles and the Monotremes, the 

 fusion of the folds above the embryo is not complete, so that 

 the amniotic cavity is not quite closed. In the birds, the 

 amniotic cavity is closed, but it opens again later (at the sero- 

 amniotic connexion). 



In the mammals, there are two principal types of amnion- 

 formation. In the one type, of which the rabbit is character- 

 istic, the embryonic plate comes to the surface of the blastocyst 

 by the disappearance of the overlying trophoblast (cells of 

 Rauber), and the amniotic folds rise up on each side of the 

 embryo from the edge of the embryonic plate. This method 

 of formation of the amnion is very similar to that which holds 

 in birds ; the chorion of the latter corresponds to the tropho- 

 blast of the mammals. The only difference is the fact that 

 the trophoblast in the mammal forms a complete investment 

 from the earliest stage. 



