THE STRUCTURE OF THE HUMAN PLACENTA. 59 



When the ovum enters the cavity of the uterus, the cellular 

 decidua surrounds it, and becomes what has been named the 

 decidua reflexa, by a continuation of the same action by which 

 it had been increasing in quantity before the arrival of the 

 ovum. The cellular decidua grows around the ovum by the 

 formation of new cells, the product of those in whose vicinity the 

 ovum happens to be situated. 



At this stage of its growth, the ovum with its external mem- 

 brane, the chorion, covered by tufts, the structure and functions 

 of which, have been described in the second pail of this Memoir, 

 is embedded in a substance which consists entirely of active 

 nucleated cells. The absorbing cells of the 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 mam- 

 mal 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 colyledons of the ruminant 

 and other mammalia as a permanent decidua vera, and the milky 

 secretion interposed between them and the foetal colyledons 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 



