73 



DEVELOPMENT 



veloped depressions or crypts in the decidua vera, which correspond in shape 

 to the villi they are to lodge; and thus the chorionic villi become more or less 

 embedded 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. 



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 communicate 

 on the one hand with arteries and on the other with veins of the uterus. Into 



Decidua basalis 

 Unchanged layer Maternal vessel 



Primitive streak 

 Mesoderm 



Placental villus 



Ectoderm 



Stratum spongiosnm 

 Stratum compactunix^ 



Placental villus 



Villus 



Cavity of 

 blastoderm 



Cavity which 

 becomes coelom 



Mesoderm 



Decidua vera-/ 



Entoderm 

 Decidua vera 



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



(Cunningham.) 



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 



