STRUCTURE OF HUMAN PLACENTA. 455 



tufts are constantly taking up either the matter resulting from 

 the solution of the cells of the cellular decidua, or the fluid 

 contained in these cells. The ovum is now deriving its 

 nourishment, not from the supply which it took along with it 

 when it left the ovary, but from a matter supplied by the 

 uterus. I am, therefore, inclined to look upon the cellular 

 decidua as representing, in the gestation of the mammal, the 

 albumen of the egg of the oviparous animal. They are both 

 supplied by a certain portion of the oviduct, and they are 

 both brought into play after the nourishment supplied by the 

 ovary is exhausted, or in the course of being exhausted. The 

 difference between them consists in this, that in the mammal 

 the albumen is applied to use as quickly as it is absorbed ; 

 whereas, in the oviparous animal, after being absorbed, it is 

 kept in reserve within the chorion till required. I have also 

 been in the habit of considering the uterine cotyledons of the 

 ruminant and other mammalia as a permanent decidua vera, 

 and the milky secretion interposed between them and the 

 foetal cotyledons as decidua reflexa in its primitive and 

 simplest form. 



I have been thus particular in the explanation of what I 

 believe to be the nutritive function performed respectively by 

 the chorion and decidua, as upon it I shall have to found my 

 views regarding the actions of nutrition in the fully developed 

 placenta. 



When the ovum has arrived at a certain stage of its growth, 

 the absorption and circulation of nutritive matter by the 

 agency of cells alone is no longer sufficient. At this period, 

 the ovum has approached the thickened mucous membrane, 

 or that portion usually described as decidua serotina. About 

 the same time, the allantois bearing the umbilical vessels 

 applies itself to the internal surface of that portion of the 

 chorion opposed to the decidua serotina, and the villi of that 

 portion become vascular, as formerly described. The vessels 



