FCETAL NUTRITION: THE PLACENTA 



469 



decidua reflexa forms the serotina, and a secondary retiexa is formed, 

 which is recognisable till the twentieth day of pregnancy. 



The increase in size of the implantation cavity is accompanied 

 by a thinning of its wall. According to Duval this is a mechanical 



sym. 



FIG. 133. Longitudinal section of the implantation cavity of the field-mouse 

 about the eighth day of pregnancy. (From Disse's " Die Vergrosserung 

 der Eikammer bei der Feldmaus (Arvicola arvalis)" Arch. f. mikr. Anctf., 

 vol. Ixviii., 1906.) 



p/, Placental pole ; mph, macrophags or giant-cells ; aym, uterine 

 symplasma ; I, blood lacuna. 



process, s,ince the cells do not increase in number, but it is probably 

 more complicated. On the inner surface of the decidua giant-cells 

 appear around the ovum, and they are phagocytic (Fig. 133). Duval 

 stated that each was derived from a cell of the foetal ectodermal 

 wall of the yolk-sac, and later from a cell of the ectoplacental cone. 

 As they increase in number, they form a distinct layer, two to five 

 cells in depth, between the yolk-sac and the wall of the implantation 



