378 



HISTOLOGY 



layer and the dark clumps of nuclei scattered over its surface may arise 

 from the syncytium, but the reverse relation of the two types of epithe- 

 lium to the original layers is sometimes stated. Frequently the villi 

 are covered in part with very conspicuous masses of hyaline material, 

 apparently derived from epithelial degeneration (Fig. 379). Deposits of a 

 substance staining deeply with eosin and resembling the fibrin of blood 

 clots may also be observed. This material is often in the form of layers, 

 with intervals between them, and is known as "canalized fibrin." It is 

 believed to be derived from the blood, but the origin of these deeply 

 staining masses is "not yet fully understood" (Stohr). 



Connective tissue. 



Hyaline substance in tan- 

 gential section. 



Blood vessel. Hyaline substance. Proliferation islands. 



FIG. 379. FROM A SECTION OF A HUMAN PLACENTA AT TERM. X 260. 



The surface of the placenta toward the embryo is covered with amnion, 

 which has remained in place in the section shown in Fig. 380. Sometimes 

 it becomes detached in preparing the specimen. It consists of homogene- 

 ous connective tissue toward the chorion, and is covered on its free surface 

 by simple low columnar epithelium, sometimes containing fat droplets and 

 vacuoles. The chorionic membrane is a much thicker layer, consisting 

 of vascular connective tissue, and covered with epithelium continuous with 

 that of the villi. The root of a villus is cut tangentially in Fig. 380. The 

 epithelium at term is often in relation with the hyaline material or "canal- 



