METAL NUTRITION: THE PLACENTA 385 



It is not yet determined whether the yolk-sac placenta is 

 functional till birth. According to Hill the wall probably breaks 

 up before the end of pregnancy. The allantoic placenta, on 

 the other hand, remains active, and at the time of birth some 



amn 



coe 



vase, 

 ompfi 



bil omph, yspl. 



FIG. 80. Diagram showing arrangement of foetal membranes in Perameles. 

 (From Hill, "The Placentation of Perameles," Quar. Jour. Micr. Sci., 

 vol xl., 1897.) 



amn, amnion ; all.c, allantoic cavity ; all.mes, allanto-chorionic mesenchyme ; 

 all.s, allantoic stalk; bilomph, bilaminar omphalopleur ; ch., marginal 

 zone of true chorion around the allanto-chorionic area ; coe, extra- 

 embryonic coelom ; coe.w, inner or chorionic wall of allantois ; proa.r, 

 persistent remnant of proamnion ; st, sinus terminalis ; vasc.omph, 

 vascular omphalopleur ; y.c, yolk-sac cavity ; y.spl., invaginated yolk- 

 sac splanchnopleur ; ectoderm represented by thin line, mesoderm by 

 dotted line, entoderm by thick line. 



maternal tissue is shed, while part of the foetal tissue is left 

 behind. The gestation period is about eight days. 



The allantois in Perameles is of greater importance than in 

 the opossum or Dasyurus ; but, relatively to the yolk-sac, it 

 plays a small part in the nutrition of the embryo, as evidenced 



2s 



