THE PLACENTA 



789 



these sinuses the villi of the chorion protrude, pushing the thin walls of the 

 sinuses before them, and so come into intimate relation with the blood con- 

 tained 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 inter- 

 vening 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 mater- 

 nal 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 

 entanglement of fetal villi and maternal sinuses, by means of which the 



Decidua basal is 

 Unchanged layer Maternal vessel 



Primitive streak 

 Mesoderm 



Placental rillus 



Ectoderm 



Stratum spongiosnm 

 Stratum compactum 



Placental villus 



Villas- 



Cavity of 

 blastoderm 



Cavity whicl 

 becomes coeloin 



Mesoderm 



Decidua vera/ 



Entodenn 

 idua vera 



FIG 504. Diagram of the Early Stage of Human Embryo in Relation to the Uterus 



(Cunningham.) 



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 

 or crushing through of that part at which its cohesion is least strong, namely, 

 where it is most burrowed and undermined by the cavernous spaces before 

 referred to. In this way it is cast off with the fetal membrane. The remain- 

 ing portion is either gradually absorbed, or thrown off in the uterine dis- 



