232 EMBRYOLOGY. 



plicated organs, which have been designated as the after-birth, or 

 placenta. Since a knowledge of them will facilitate our compre- 

 hension of the human placenta, we shall consider them somewhat at 

 length. 



It is most expedient to distinguish three different modifications in the 

 way in which the surface of the blastodermic vesicle comes into relation 

 with the mucous membrane of the uterus, and accordingly to divide the 

 Mammals into three groups. 



In one the serosa is retained nearly in its simple primitive condition, 



In the second it is transformed into a villous layer or chorion, and 



In the third a placenta arises out of one or more portions of the chorion. 



To the first group belong, among the Mammalia, only the Mono- 

 tremes and the Marsupials, whose embryonic membranes are in the 

 main constituted like those of Birds and Reptiles. Ordinarily in the 

 Marsupials the serosa retains its smooth surface. Inasmuch as it 

 lies in close contact with the vascular mucous membrane of the uterus, 

 it can absorb nourishment from the latter and transmit it to the 

 deeper-lying embryonic parts. 



In the second group of Mammals an improvement in the intra- 

 uterine nourishment is effected by important changes in the organisa- 

 tion of the serosa, which is converted into a villous layer or chorion. 



In the first place, it is provided with blood-vessels by the allantois, 

 which grows out into contact with it, and whose connective-tissue 

 layer, containing the ramifications of the umbilical vessels, grows 

 over its entire inner surface. 



Secondly, the epithelial membrane begins to grow out into folds 

 and villi, into which there soon penetrate vascular outgrowths of the 

 connective-tissue layer. By this process a larger resorbing surface 

 is provided. 



Thirdly, the mucous membrane of the uterus and the chorion 

 unite more intimately and firmly with each other, while the former 

 also increases its surface and acquires pits and depressions into which 

 the processes of the latter penetrate. 



All these changes have simply the purpose of facilitating and 

 rendering more perfect the interchange of materials between the 

 tissues of the mother and those of the offspring. 



We meet with membranes thus constituted in the Suidse, the 

 Perissodactyla, Hippopotamidae, Tylopoda, Tragulidae, Sirenia, and 

 Cetacea. In the Pig, which shall serve as an example, the blasto- 

 dermic vesicle, in adaptation to the form of the uterus, is transformed 

 into a spindle-shaped sac. The inner embryonic appendages, the 



