754 ' 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 228 * contained. The effect of 



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, and in which soon afterwards three portions are dis- 

 tinguishable. These have been named the decidua vera, the 

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

 the decidua vera, lines the cavity of the uterus ; the second, 

 or decidua reflecca, 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, 

 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 fcetal 

 part of the placenta. 



* Fig. 228. 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). 



