240 



THE UROGENITAL SYSTEM 



occur syncytial giant cells to be derived from the trophoderm of the villi. 

 It is in the plane of this spongy layer that the separation of the placenta 

 takes place at birth. The decidua is said to prevent excessive haemor- 

 rhage during the earlier part of pregnancy by acting as a dam between the 

 chorionic villi and the eroded uterus (cf. p. 235). 



'yncytium 



Cuboidal cell~^(of Langhans) 



'onnedive tissue 



Blood vessel containing 

 nucleated red corpuscles 



Oblique section of the epithelium 



Epithelium" 

 Epithelial nucleus 



Capillaries *~ - - - 



Syncytial knot 



Small artery 



Syncytial knot - 



Syncytial knot 



Epithelium 



--.-W-j~Small -vein 



Capillary 



FIG. 248. Transverse sections of chorionic villi: A, at the fourth week; B, C, at the end of 

 pregnancy (Schaper in Lewis and Stohr). 



The basal plate, or compact layer of the decidua basalis, is composed of 

 a connective-tissue stroma containing decidual cells, canalized fibrin, 

 and persisting portions of the epithelium of the villi. The 'canalized 

 fibrin' (Fig. 242) forms chiefly by a fibrinoid necrosis of the mucosa, but 



