694 



DEVELOPMENT 



portant to note, are not, as was once supposed, merely the open mouths of 

 the uterine follicles. 



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 hese sinuses the villi of the chorion protrude, pushing the thin 



Pecidua basalis 

 Unchanged layer Maternal vessel 



Stratum spongiosum 

 Stratum compactum 



Placontal villus. 



Primitive streak 

 Mesoderm 



Placental villus 



Cavity wind 

 becomes cud on 



Decidua vera/ 



Decidua vera 



FIG. 502. Diagram of the Early Stage of Human Embryo in Relation to the Uterus. 



(Cunningham.) 



walls of the sinuses 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 fetus; 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 by diffusion and osmosis. Thus 

 the villi of the chorion, containing fetal blood, are bathed or soaked in maternal 

 blood contained in the uterine sinuses. 



The placenta, therefore, of the human subject is composed of a fetal part 

 and a maternal part the term placenta properly including all that entangle- 

 ment of fetal villi and maternal sinuses, by means of which the blood of the 

 fetus is enriched and purified after the fashion necessary for the proper growth 

 and development of those parts which it is designed to nourish. 



The whole of this structure is not, as might be imagined, thrown off 

 immediately after birth. The greater part, indeed, comes away at that time, 

 as the after-birth; and the separation of this portion takes place by a rending 



