THE PLACENTA IN SITU.' 



355 



a cell patch may form the whole thickness of the ectoderm, as shown in figure 235, 

 or the whole of a cell patch may do so. More commonly, however, the cellular 

 patch is covered more or less completely by a 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 chorionic ectoderm, but it may also spread into 



FIG. 236. HUMAN CHORION OF SEVEN MONTHS' PLACENTA. 

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



the' cellular layer, which then becomes replaced by fibrin, so that this last alone 

 represents the ectoderm of the chorion. . The fibrin layer consists of a very refrin- 

 gent substance permeated by numerous channels (Fig. 236, fb). The substance has 

 a violent affinity for carmine and hematoxylin, and hence is always deeply colored 

 in sections stained with either of these dyes. The channels tend to run more or 

 less parallel to the surface of the chorion, and are connected by numerous 

 short cross-channels. Some of the channels contain cells or nuclei. The appear- 

 ances, however, are very variable; the fibrin often sends long outshoots into the 



