FCETAL NUTRITION: THE PLACENTA 



469 



decidua reflexa forms the serotiua, and a Heoondaiy retlexa is formed, 

 which is recognisable till the twentieth day of pregnancy. 



The increase in size of the implantation cavit}' is accompanied 

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



sym. 



Kitt. 133. — Longitudinal sectiun uf the implantation cavity of the held-uiou.se 

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

 der Eikammei- Ijei der Feldiuaus f.lrc/roi'ff a rvaf ix)," Arch. f. rnihr. Ajuif., 

 vol. Ixviii., 1906.) 



J)/, Placental pole ; iihp/i, macrophag.s or giant-celts ; •<////', uterine 

 symplasma ; /, Ijlood lacuna. 



process, since the cells do not increase in iunnl:ier, Imt it is ])ri>lialily 

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

 a,ppear around the o\'um, and they are phagdcytie (Fig. l.SoV Dnval 

 stated that each was derived from a cell of the fVetal ectoderm.d 

 wall of the yolk-sac, and latei- from a cidl of the ectoplacental cone 

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

 cells in deptli, lietween tlie yolk-sac and the wall of tlie implaidation 



