DECIDUAL MEMBRANES 375 



cavernous layer of the mucosa contains slender clefts parallel with the 

 muscularis. These are glands which have been stretched laterally; some 

 of them retain areas of normal epithelium, but in many the epithelium has 

 degenerated, and from some it has wholly disappeared. The connective 

 tissue is but slightly modified. Throughout the decidua, but especially 

 in the superficial portion, the vessels are greatly distended. 



PLACENTA. 



The chorionic villi, the interlacing branches of which form the fetal 

 portion of the placenta, are shaped as in Fig. 375. The finding of such 

 structures in a uterine discharge or curetting is diagnostic of preg- 

 nancy. The villi in the earliest stages are composed entirely of epi- 

 thelium, but they soon acquire a core of the chorionic mesenchymal tissue, 

 in which are the terminal branches of the umbilical vessels. The epi- 

 thelium is very early divisible into two layers. The outer layer consists 



XI9 



FIG. 37S- ISOLATED TERMINAL BRANCHES OF CHORIONIC VILLI; THAT ON THE LEFT is FROM AN EMBRYO 

 OF TWELVE WEEKS; ON THE RIGHT, AT FULL TERM. (Minot.) 



of densely staining protoplasm, said to contain fat granules and to pre- 

 sent a brush border; it has dark, round or flattened nuclei. Since cell 

 boundaries are lacking, this is called the syncytial layer (Fig. 376). 

 Mitotic figures are seldom seen in it. Generally its nuclei are in a single 

 layer at varying distances from one another, but they may accumulate 

 in "knots" or "proliferation islands," especially in late stages (Fig. 377). 

 The knots project from the surface of the villi, so that in certain planes 

 of section they appear completely detached and suggest multinucleate 

 giant cells. The syncytial layer perhaps completely invests the villi at 

 first, but later it is interrupted in many places. 



