DEVELOPMENT. 



675 



vessels are contained. The effect of these changes is an increased 

 thickness, softness, and vascularitj of the mucous membrane, the super- 

 ficial part of which itself forms the membrana decidua. 



The object of this increased 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 imbedded. 



When the ovum first enters the uterus it becomes imbedded in the 

 structure of the decidua, which is yet quite soft, and in which soon 

 afterwards three portions are distinguishable. 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 reflexa, is a part of the decidua vera which grows up around 

 the ovum, and, wrapping it closely, forms its immediate investment. 



FIG. 462. 



FIG. 463. 



FIG. 462. Two thin segments of human decidua after recent impregnation, viewed on a dark 

 ground : they show the openings on the surface of the membrane. A, is magnified six diameters, and 

 .a, twelve diameters. At 1, the lining of epithelium is seen within the orifices, at 2 it has escaped. 

 (Sharpey.) 



FIG. 463. Diagram of an early stage of the formation of the human placenta, a, embryo; 6, 

 amnion; c, placental vessels; d, decidua reflexa; e, allantois; /, placenta! villi; g, mucous mem- 

 brane. CCadiat.) 



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 foetal part 

 of the placenta. 



In connection with these villous processes of the chorion, there are 

 developed depressions or crypts in the decidual mucous membrane, which 

 correspond in shape with the villi they are to lodge; and thus the chori- 

 onic villi become more or less imbedded in the maternal structures. 

 These uterine crypts, it is important to note, are not, as was once sup- 

 posed, merely the open mouths of the uterine follicles. 



As the ovum increases in size, the decidua vera and the decidua reflexa 



