THE PLACENTA IN SITU. 



331 



muscular coat, Me, of the uterus. The ends of some of the villi touch, and are 

 imbedded in, the decidual tissue. Their imbedded ends are without covering 

 epithelium, but their connective tissue is immediately surrounded by hyaline 

 substance which is probably degenerated epithelium. The decidua serotina is 

 plainly divided into an upper compact, D f , and a lower cavernous layer, D" . The 

 section figured passes through an arterial vessel, ve, which makes an abrupt turn 

 so as to discharge its blood into the intervillous spaces. 



The histological structure of all the parts should be carefully studied. (As 

 regards the structure of the amnion, see page 349.) 



The chorion consists of two layers, the outer ectodermic and inner meso- 

 dermic. Over the chorionic membrane proper the ectoderm offers a great variety 



Am. 



FIG. 191. HUMAN PI.ACENTAL CHORION AND AMNION OF THE FIFTH MONTH. 



Ep, Amniotic epithelium. Am, Amnion. Str, Stroma. Fib, Fibrillar layer. Fbr, Fibrin layer, c, Chorionic 

 cellular layer of ectoderm. Vi, Chorionic villi. X 7 1 diams. 



of appearances. In some places it may be seen to have still its primitive organi- 

 zation, a single inner layer of distinct cells and an outer syncytial layer, more or 

 less similar to those represented in figure 199. For the most part, however, the 

 chorionic ectoderm has been considerably modified from its primitive condition. 

 The inner or cellular layer exhibits irregular, thickened patches, which present 

 every possible degree of variation as to their size. A cell patch from a somewhat 

 younger stage is represented in figure 191 as seen with a low magnification, and 

 another patch of the age we are studying is represented in figure 192. The 

 patches vary in appearance ; the cells are more distinct in the small patches, less 



