616 



OF REPRODUCTION. 



and its extent bears a pretty close correspondence with the extent of surface, 

 through which the Chorion comes into vascular connection with the Decidua. 

 Thus, in the Carnivora, whose Placenta extends like a band around the whole 

 Ovum, the Allantois also lines the whole inner surface of the Chorion, except 

 where the Umbilical Vesicle comes in contact with it. On the other hand, in 

 Man and the Quadrumana, whose Placenta is restricted to one spot, the Allan- 

 tois is small, and conveys the foetal vessels to one portion only of the Chorion. 

 When these vessels have reached the Chorion, they ramify in its substance, 

 and send filaments into its villi ; and in proportion as these villi form that con- 

 nection with the uterine structure which has been already described, do the 

 vessels increase in size. They then pass directly from the Foetus to the 

 Chorion ; and the Allantois being no longer of any use, shrivels up, and 

 remains as a minute vesicle, only to be detected by careful examination. The 

 same thing happens in regard to the Umbilical vesicle, from which the entire 

 contents have been by this time exhausted ; and from henceforth the Fretus is 

 entirely dependent for the materials of its growth, upon the supply it receives 

 through the Placenta, which is conducted to it by the vessels of the Umbilical 



Diagram of Human Ovum, at the time of formation of the placenta; a, muco-gelalinous substance, block- 

 ing up os uteri ; 6, b, Fallopian tubes; e, c, decidua vera, prolonged at c 2, into Fallopian tube ; el, cavity of 

 uterus, almost completely occupied by ovum (compare with fig. 161) ; e, e, angles at which decidua vera is 

 reflected;/, decidua serotina; g, allantois; A, umbilical vesicle; i, amuion; &, chorion, lined with outer 

 fold of serous tunic. (After Wagner.) 



