PLACENTA (FUNCTIONS). 



Fig. 487. 



721 



The formation of the foetal chamber more advanced than in Jig. 486. The ovum is still evenly covered by villi, 

 the ends of which, about this time, begin to be attached to the walls of the chamber in which the ovum is 

 contained. 



o, ovum ; d r, decidua reflexa, forming the foetal chamber; dv, decidua vera; , uterine walls. 



ing the formation of a subcutaneous abscess j 

 while in proportion as the base of the chamber 

 becomes extended by the gradual retiring, from 

 the centre, of the line of reflexion of the de- 

 cidua, like waves receding from a central point, 

 so, at the same time, an increasing surface is 

 produced by the expansion of the uterus itself; 

 and the layer of decidua here formed, com- 

 monly termed the decidua serotina, is simply 

 the mucous membrane reproduced to supply 

 the place of that which had been consumed 

 or pushed off in forming the decidua reflexa. 



The fourth or final stage consists in the 

 partitioning of the permanent placenta into 

 smaller portions or lobes by the extension of 

 the layer of decidua (serotina) which lies op- 

 posite to the developed villi inwards at vari- 

 ous points towards the chorion. In this way 

 are constructed the dissepiments already de- 

 Supp. 



scribed as bounding the several lobes or co* 

 tyledons. This partitioning of the placenta 

 commences about the fourth month. 



Thus, during these several stages in the 

 formation of the placenta, two processes may 

 be said to be concurrently carried on which 

 tend in opposite directions a process of 

 positive enlargement and growth combined 

 with one of relative retrogression or limita- 

 tion. For while the bulk of the placenta is 

 progressively increasing up to the completion 

 of pregnancy, the relative amount of surface 

 of the ovum appropriated to it is, on the other 

 hand, diminished. The entire surface of the 

 chorion being, in the first stages of develop- 

 ment, employed as a placenta ; while in the 

 latter half of gestation, one third of it suffices 

 for that purpose. 



Function* of the placenta. By means of 

 3 A 



