THE PLACENTA. 757 



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 difficult, or even 

 impossible to distinguish the two layers. 



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 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 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 inter- 

 change 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 con- 

 taining maternal blood. But in the foetal villi there is a 

 constant stream of blood into and out of the loop of capil- 

 lary blood-vessel contained in it, as there is also into and 

 out of the maternal sinuses. 



It would seem from the observations of Professor 

 Goodsir, that, at the villi of the placental tufts, where the 

 foetal and maternal portions of the placenta are brought 

 into close relation with each other, the blood in the vessels 

 of the mother is separated from that in the vessels of the 

 foetus by the intervention of two distinct sets of nucleated 

 cells (fig. 223). One of these (&), belongs to the maternal 

 portion of the placenta, is placed between the membrane of 

 the villus and that of the vascular system of the mother, 

 and is probably designed to separate from the blood of the 



