MAMMALS. 373 



Like the other amniotes the mammalian embryo forms the 

 foetal structures amnion, serosa, and allantois (p. 288). Of 

 these the serosa is the outermost, and necessarily comes in con- 

 tact with the uterine walls. In most marsupials the develop- 

 ment goes little farther. From the uterus is secreted nutrient 

 fluid which passes through the serosa by osmosis, and is thence 

 taken up by the embryo, furnishing it with the material for 

 growth, which in oviparous forms is supplied by the yolk. 



In Perameles, one of the marsupials, and in all the higher 

 mammals, a more intimate union occurs between the embryo 

 and the uterine walls in the following manner. From the sur- 

 face of the serosa (which from this time on is known as the 

 chorion) numerous outgrowths or villi are formed. These villi 

 are variously arranged in different mammals. They may be 

 distributed evenly over the whole chorionic surface (diffuse), or 

 they may be collected in tufts scattered over the surface, the 

 intermediate regions of the chorion being smooth (cotyledonary)- ; 

 again, they may form a girdle around the chorion, the ends being 

 free from villi (zonary) ; or, lastly, they may be restricted to a 

 more or less circular patch on one side of the chorion (discoidal). 

 These villi enter into more or less intimate connection with the 

 uterine walls in ways to be described below. 



The allantois (p. 289) grows out from the body, and finally 

 reaches the inner surface of the chorion, carrying with it the 

 allantoic blood-vessels. The union of chorion and allantois is 

 coextensive with the development of the villi upon the outer 

 surface, and the resulting structure forms the embryonic por- 

 tion of the placenta. The blood-vessels of the allantois may be 

 confined to that structure, or they may extend out into the cho- 

 rion, but in either case they carry away from the embryo waste 

 which passes, by osmosis, to the maternal tissues, and at the 

 same time bring back to the growing young nourishment and 

 oxygen, which pass into the foetal blood by osmotic action. In 

 no case is there a direct connection between maternal and foetal 

 blood-vessels ; but the exchange is always of the character 

 indicated here. 



It must, however, be noted that the relations of the allantois 

 to the chorion follow two types. In the unguiculate mammals 



