756 GENERATION AND DEVELOPMENT. 



the mucous membrane become larger and more numerous, 

 while a substance composed chiefly of nucleated cells fills 

 up the interfollicular spaces in which the blood-vessels are 

 Fig. 222.* contained. The effect of 



B these changes is an in- 



creased thickness, softness, 

 and vascularity of the mu- 

 cous membrane, the super- 

 ficial part of which itself 

 forms the membrana de- 

 cidua. 



The object of this in- 

 creased development seems 

 to be the production of 

 nutritive materials for the 



ovum ; for the cavity of the uterus shortly becomes filled 

 with secreted fluid, consisting almost entirely of nucleated 

 cells, in which the villi of the chorion are embedded. 



When the ovum first enters the uterus it becomes im- 

 bedded in the structure of the decidua, which is yet quite soft, 

 a*nd in which soon afterwards three portions are distinguish- 

 able. These have been named the decidua vera, the decidua 

 reflexa, and the decidua serotina. The first of these, the 

 decidua vera (3, fig. 219), lines the cavity of the uterus ; the 

 second, or decidua reflexa (<;), is a part of the decidua vera, 

 which grows up around the ovum, and, wrapping it closely, 

 forms its immediate investment. The third, or decidua 

 serotina (7), is the part of the decidua vera which becomes 

 especially developed in connection with those villi of the 

 chorion which, instead of disappearing, remain to form 

 the foetal part of the placenta. 



Fig. 222. Two thin segments of human decidua after recent im- 

 pregnation, viewed on a dark ground : they show the openings on the 

 surface of the membrane. A is magnified six diameters, and B twelve 

 diameters. At i, the lining of epithelium is seen within the orifices, at 

 2 it has escaped (from Sharpey). 



