THE PLACENTA. 591 



foil icular spaces in which the bloodvessels are contained. The 

 effect of these changes is an increased thickness, softness, and 

 vascularity of the mucous membrane, the superficial 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, consist- 

 ing 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. The 

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

 becomes especially developed in connection with those villi of 

 the choriori which, instead of disappearing, remain to form the 

 foetal part of the placenta. 



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 disap- 

 peared. Henceforth it is very difficult, or even impossible, to 

 distinguish the two layers. 



During these changes the deeper part of the mucous mem- 

 brane of the uterus, at and near the region where the placenta 

 is placed, becomes hollowed out by sinuses, or cavernous spaces, 

 which communicate 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 con- 

 tained in them. There is no direct communication between 

 the bloodvessels 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, con- 

 taining 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 bloodvessel contained in 

 it, as there is also into and out of the maternal sinuses. 



It would seem from the observations of Professor Goodsir, 



