268 HAND-BOOK OF PHYSIOLOGY. 



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 after- 

 ward 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 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 supposed, 

 merely the open mouths of the uterine follicles (Turner). 



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

 gradually come into contact, and in the third month of pregnancy the 

 cavity between them has quite disappeared. Henceforth it is very diffi- 

 cult, or even impossible, to distinguish the two layers. 



The Placenta. During these changes the deeper part of the mucous 

 membrane of the uterus, at and near the region where the placenta is 

 placed, becomes hollowed out by sinuses, or cavernous spaces, which com- 

 municate on the one hand with arteries and on the other with veins of 

 the uterus. Into these sinuses the villi of the chorion protrude, pushing 

 the thin wall of the sinus before them, and so come into intimate relation 

 with the blood contained in them. There is no direct communication 

 between the blood-vessels of the mother and those of the foetus; but the 

 layer or layers of membrane intervening between the blood of the one and 

 of the other offer no obstacle to a free interchange of matters between 

 them. Thus the villi of the chorion containing foetal blood, are bathed 

 or soaked in maternal blood contained in the uterine sinuses. The 

 arrangement may be roughly compared to filling a glove with foetal 

 blood, and dipping its fingers into a vessel containing maternal blood. 

 But in the foetal villi there is a constant stream of blood into and out of 

 the loop of capillary blood-vessels contained in it, as there is also into and 

 out of the maternal sinuses. 



It would seem from the observations of Goodsir, that, at the villi 

 of the placental tufts, where the foetal and maternal portions of the 



