DEVELOPMENT. 



785 



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 chorion villi 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 

 afterward 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. 



The third, or decidua serotina, is the part of the decidua vera which 

 becomes especially developed in connection with those villi of the cho- 

 rion, 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. 



Fig. 482. Diagram of an early stage of the formation of the human placenta, a, embryo ; 

 6, amnion ; c, placental vessels ; d, decidua reflexa ; e, allantois ; /, placental villi ; a, mucous 

 membrane. (Cadiat.) 



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 gradually come into contact, and in the third month of preg- 

 nancy the cavity between them has almost disappeared. Though the 

 two layers come into contact at the third month, they are not closely 

 amalgamated until the end of the sixth month. 



The Placenta. During these changes the deeper part of the mu- 

 5 



