FCETAL NUTRITION: THE PLACENTA 419 



part of the wall is bathed by the uterine fluid as in the opossum 

 (Fig. 105). 



In the discoid area a factional allantoic placenta is developed. 

 The ectodermal giant-cells, Uke the early trophoblastic proliferation 

 in man, disappear, and the allantoic vessels become firmly attached 

 to the symplasma into which the maternal vessels penetrate. A 

 regular interlocking of maternal and foetal tissues is produced, and 



amn. 



coe 



til amph. y spL. 



Fig. 105. — Diagram showing arrangement . of foetal membrar^^ in Perameles. 

 (From Hill, "The Placentation of Perameles" Quar. JoUr. Micr. Science, 

 vol. xl., 1897.) 



omm. Amnion ; all.c, allantoic cavity ; all.ines, allanto-chorionic mesenchyme ; 

 all.s, allantoic stalk ; bil.omph, bilaminar omphalopleur ; ck, marginal zone 

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

 ccelom ; coe.w, inner or chorionic wall of allantois ; proa.r, persistent 

 remnant of proamnion ; gt, sinus terminalis ; vasa.omph, vascular omphalo- 

 pleur ; y.c, yolk-sac cavity ; y.spl, invaginated yolk-sac splanchnopleur ; 

 ectoderm represented by thin line, mesoderm by dotted line, entoderm by 

 thick line. . 



the two systems of blood-vessels are separated at the most by a thin 

 layer of symplasma. 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 

 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 



