FCETAL NUTRITION: THE PLACENTA 471 



origin, the mononuclear cells in Man appear to be engaged in 

 disintegrating mucosal tissue, and producing a zone of coagula- 

 tion necrosis, i.e, a symplasma, around the trophoblast. But 

 they differ from similarly situated, cells in lower animals, e.g. 

 the mouse, in showing no evidence of ingestion of formed tissue- 

 elements. 



Fici. 124. Section of a portion of the necrotic zone of the decidua, and of 

 the layer of large cells ou its inner aspect. (Bryce and Teacher.) 



nz., necrotic zone ; me., large cells in various stages of degeneration ; 

 cav., blood- filled implantation cavity. 



In the youngest ova no space exists between the trophoblast 

 and the wall of the implantation cavity (Fig. 125). In later 

 specimens a space is formed, apparently by the absorption of the 

 debris of the necrotic zone. How this excavation is brought 

 about is uncertain. According to Peters, the trophoblast may 

 exercise a phagocytic action. Bryce and Teacher, however, found 



