UROGENITAL SYSTEM. 351 



mals the umbilical or navel cord, in which the blood-vessels run. In 

 the mammals there are several variations from the above account of the 

 development of the allantois, but they can be reconciled with the 

 typical condition in the sauropsida. There are also several other 

 variations and the relations of allantois to the other structures is more 

 complicated, but details and the many modifications must be ignored 

 here, only an outline of the broader features being given. 



In the mammals there is the same fusion of allantois and serosa as 

 in the sauropsida, the fused area here being called the chorion. On 

 arrival in the uterus by way of the Fallopian tube, the egg becomes 

 implanted in the uterine wall, and a little later, with the development 

 of the chorion, villi are formed on the outer surface of the egg. These 

 are invaded by the chorionic blood-vessels and they branch and extend 

 into depressions or crypts in the walls of the uterus. The latter become 

 very vascular, the blood spaces of the maternal tissue enveloping the 

 villi with only the thinnest of walls between the vessels of the mother 

 and those of the young. (There is never any actual connexion between 

 the blood-vessel of parent and embryo and so blood corpuscles cannot 

 pass from one to the other. All that takes place is largely of the nature 

 of osmosis solutions of gases, of nourishing substances and of nitrog- 

 enous waste passing from one to the other. There is difficulty in 

 explaining the passage of proteids and fats.) This structure, consisting 

 of the allantoic derivatives of the embryo and the mucous lining of the 

 uterus, is known as the placenta. 



In the monotremes and in most marsupials no placenta is formed, 

 but it has been recently shown that a true placenta occurs in a few of the 

 latter group. In other mammals a placenta always occurs, the struc- 

 tures presenting many forms, but these may be grouped under a few 

 heads. (It must be borne in mind that this classification is purely 

 morphological and does not necessarily imply close relations of the 

 species included or identity of method of formation.) 



In many mammals, at the time of birth, the maternal and embryonic 

 parts of the placenta simply separate, only the latter passing away 

 with the young. These are called non-deciduate placentae. In 

 the others the union of the fcetal and the maternal tissues is so intimate 

 that the inner surface of the uterus is included in the afterbirth. These 

 form the deciduate type. The non-deciduata include two divisions. 

 In the diffuse placentas (edentates, whales, perissodactyls, many 

 artiodactyls) the villi are distributed over the entire surface of the 



