332 



HUMAN UTERUS AND FCETAL APPENDAGES. 



so in the large patches, in which there are often parts more or less degenerated. 

 The cell-bodies stain lightly; their nuclei are granular, not very sharply defined, 

 and variable in size and shape. The cellular layer is always sharply defined 

 against the mesoderm. Toward the outside the patches offer varying relations. 

 In some cases a part of a cell patch may form the whole thickness of the ecto- 

 derm, as shown in figure 191, or the whole of a cell patch may do so. More 

 commonly, however, the cellular patch is covered more or less completely by a 



^*s^* 



^^■' 



ines 



Fig. 192. — Human Chorion of Seven Months' Placenta. 

 c, Cellular layer, fb. Fibrin layer, ep. Remnants of epithelial layer. meSy Mesoderm. X 445 diams. 



special substance, which is termed canalized fibrin, and which is believed to 

 represent the original outer syncytial layer in a degenerated condition. The 

 fibrin is a constant, normal, and very remarkable constituent of the placenta. 

 Its formation seems to begin always in the outer or syncytial layer of the cho- 

 rionic ectoderm, but it may also spread into the cellular layer, which then be- 

 comes replaced by fibrin, so that this last alone represents the ectoderm of the 



