PLACENTA. 



307 



So extraordinary is this that attempts have been made to detect an endo- 

 thelial covering for the villi, but without success. (The syncytial layer 

 has been considered endothelial or otherwise of maternal origin, but this 

 view is not accepted.) 



The placenta at birth, being an inch thick, presents in cross section a 

 vast number of the branches of villi cut in various planes. In the villi 

 of Fig. 352 it is seen that the epithelium is in places hardly distinguishable 

 from the connective tissue. This thin portion may represent the basal 

 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. Within the villus are the 



blood vessels of the embryo; their blood never mixes with the maternal 



A 



Epithelium. --- 

 Epithelial nucleus 



Capillaries. <^ - - - - 



Syncytial knot. 



SmalPartery 



Syncytial knot. 



Syncytial knot. 



. Epithelium. 



ia.-Small vein. 



_ _, i Capillary. 



FIG. 352. CROSS-SECTION!THROUGH A SMALLER (A) AXD A LARGER (B) CHOKIONIC VILLUS OF A HUMAN 



PLACENTA|AT t THE END OP PREGNANCY. X 250. (Schaper.) 



blood which surrounds the villi, as is easily seen in the early stages when 

 the fetal blood contains nucleated red corpuscles. 



The embryonic surface of the placenta is shown in Fig. 353. Along 

 the .chorionic epithelium there are generally areas of hyaline material 

 w r hich stain deeply with eosin and have the appearance of fibrin. In the 

 outer part of the placenta also, the villi may seem to terminate in hyaline 

 masses attributable to the degeneration of the inner epithelium. The 

 hyaline masses [canalized fibrin] are a conspicuous feature of the mature 

 placenta. 



The decidua basalis consists of 'compact and cavernous layers, thin- 

 ner but similar to those of the vera (Fig. 354). It sends septa into the 

 fetal part of the placenta dividing it into coarse lobes or "cotyledons." 



