428 



THE FEMALE REPRODUCTIVE ORGANS 



ively forming the trophoblast, together with masses of canalized 

 fibrin. 



The trophoblast consists of large ovoid cells. It presents a 

 homogeneous appearance, not unlike that of the maternal decidua 

 serotina, and is derived from the fetal ectoderm. Its cells form 



FIG. 345. HUMAN PLACENTAL TISSUES (AMNION AND CHOBION) AT THE FIFTH MONTH. 



Ep, epithelium of the amnion ; Am, amnion ; c, cellular layer of the chorion ; Fib, fibrillar 

 layer; Fbr, canalized fibrin ; Str, stroma ; Fi, chorionic villi. x 71. (After Minot.) 



the larger part of the outer portion of the membranous chorion 

 and are continued into the main stems of the chorionic villi. In 

 the early months of pregnancy they occur in all the primitive 

 villi of the placenta, where they are found on the surface of the 

 connective tissue core, beneath the syncytium, and are known as 

 the cells of Langerhans. Later they appear to degenerate, and are 

 of less frequent occurrence in the chorionic villi. 



The canalized fibrin is of doubtful origin. It forms irregular 

 plate-like masses which either invade the substance of the mem- 

 branous chorion or here and there clothe the placental surface of 

 the mass of trophoblastic cells. Occasionally, and especially upon 

 the surface of the chorionic villi, it apparently replaces portions 

 of the syncytial membrane which elsewhere covers the villi, lines 

 the maternal surface of the membranous chorion, and therefore 

 forms the proper wall of the intervillous or maternal blood spaces. 



