754 DEVELOPMENT OF THE DECIDUAL MEMBRANE. 



Fig. 269. 



is thus provided, corresponding in abundance with the greater size of 

 the egg. 



Yery soon the activity of absorption is still further increased. The 

 chorion becomes vascular, by the formation of bloodvessels emerging 

 from the body of the embryo and penetrating everywhere into the villo- 

 sities with which it is covered. Each villosity then contains a vascular 

 loop, imbedded with itself in the substance of the decidua, and serving 

 to absorb from the uterine, mucous membrane the materials for the 

 growth of the embryo. 



Subsequently, the vascular tufts of the chorion, which are at first uni- 

 formly distributed over its surface, disappear throughout the greater 



part of its extent, while they become still 

 further developed and concentrated at a 

 particular point, the situation of the 

 future placenta. This is the spot at 

 which the egg is in contact with the de- 

 cidua. Here, both the decidual membrane 

 and the tufts of the chorion continue to 

 increase in thickness and vascularity ; 

 while elsewhere, over the prominent por- 

 tion of the egg, the chorion not only be- 

 comes bare of villosities and compara- 

 tively destitute of bloodvessels, but the 

 decidua reflexa, which is in contact with 

 it, also loses its activity of growth and 

 becomes expanded into a thin layer, with- 

 out any remaining trace of glandular fol- 

 licles. 



The uterine mucous membrane is there- 

 fore developed, during gestation, in such a way as to provide for the 

 nourishment of the embryo in the different stages of its growth. At first, 

 the whole of it is uniformly increased in thickness (decidua vera). Next, 

 a portion of it grows upward around the egg, and covers its projecting 

 surface (decidua reflexa). Afterward, both the decidua reflexa and the 

 greater part of the decidua vera diminish in the activity of their growth, 

 and lose their importance as a means of nourishment for the embryo ; 

 while that part which is in contact with the vascular tufts of the chorion 

 continues to grow, becoming excessively developed, and taking part in 

 the formation of the placenta. 



PREGNANT UTERUS; showing 

 the formation of the placenta by 

 the united development of a portion 

 of the decidua and the villosities of 

 the chorion. 





