DECIDUAL MEMBRANES 



377 



escapes into the intervillous spaces, bounded by the syncytium, or where 

 this is deficient, by the basal cells. The maternal blood circulates in the 

 intervillous spaces as shown in the diagram Fig. 378, and does not clot. 

 So extraordinary is this, that attempts have been made to detect an endo- 

 thelial covering for the villi, but without success. (The syncytial layer 

 has been considered endothelial or otherwise of maternal origin, but this 

 view is not accepted.) It is said that the products of the disintegration 

 of the maternal tissue, including haemoglobin and even entire red cor- 

 puscles, are taken up by the syncytium and used for the nutrition of the 

 embryo. 



Chorionic villi.. 



Decidua 

 basalis. 



Compact 

 layer. 



Cavernous , 

 layer. 



Muscularis. 



Vein. 



FIG. 378. DIAGRAM OF THE HUMAN PLACENTA AT THE CLOSE OF PREGNANCY. (Schaper.) 



The placenta at birth, being an inch thick, presents in cross section a 

 vast number of the branches of villi cut in various planes. A small frag- 

 ment is shown in Fig. 379. On the left, there is a section of a large villus, 

 containing fibrous tissue of the loose embryonic type, in some cases form- 

 ing a thin basement membrane beneath the epithelium. Each villus 

 contains a branch of the umbilical artery which ends in capillaries of very 

 large but varying caliber. They are situated close beneath the epithelial 

 layer, through which nutriment is transferred from the maternal blood 

 in the intervillous spaces to that of the embryo in the vessels of the villi. 

 Maternal and fetal blood never mingle, as may readily be seen in early 

 stages when the embryonic blood contains nucleated red corpuscles. 



The two primary layers of the chorionic epithelium are difficult to 

 recognize in many parts of the placenta at birth. Thus in the villi shown 

 in Fig. 377 it is seen that the epithelium is in places hardly distinguishable 

 from the connective tissue. This thin portion may represent the basal 



