620 DEVELOPMENT OF UTERINE MUCOUS MEMBRANE. 



Fig. 223. 



disappear over a portion of its surface, while they at the same time 

 become concentrated and still further developed at a particular 

 spot, the situation of the future placenta. (Fig. 223.) This is the 



spot at which the egg is in contact with 

 the decidua vera. Here, therefore, both 

 the decidual membrane and the tufts 

 of the chorion continue to increase in 

 thickness and vascularity ; while else- 

 where, over the prominent portion of 

 the egg, the chorion not only becomes 

 bare of villosities, and comparatively 

 destitute of vessels, but the decidua re- 

 flexa, which is in contact with it, also 

 loses its activity of growth, and be- 

 comes expanded into a thin layer, nearly 

 destitute of vessels, and without any 

 remaining trace of tubules or follicles. 

 The uterine mucous membrane is 

 therefore developed, during the process 

 of gestation, in such a way as to provide 



for the nourishment of the foetus 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 reflex a). 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 egg ; while that part which is in contact with 

 the vascular tufts of the chorion continues to grow, becoming ex- 

 ceedingly developed, and taking an active part in the formation of 

 the placenta. 



In the following chapter, we shall examine more particularly the 

 structure, and development of the placenta itself, and of those parts 

 which are immediately connected with it. 



PREGNANT UTERUS; showing 

 formation of placenta, by the united 

 development of a portion of the de- 

 cidua and the villosities of the cho- 

 rion. 



