THE TLACENTATION OF PERAMELES. 411 



ferent mammals has been already discussed by Semou (8). 

 He has come to the conclusion that it has an important phy- 

 siological meaning; in his own words (p. 55), " hier erfolgt 

 eben der Durchtritt der von der Mutter gelieferten Nahrungs- 

 stofFe in das Innere des Dottersacks, von wo aus weiterhin die 

 Aufnahme und Uebergabe an das Blut durch die Entoderm- 

 zellen der gefiisshaltigen Zone ausgefiihrt wird.'^ We may, 

 however, point out that in Perameles only a small proportion 

 of the uterine secretion need take this indirect way of reach- 

 ing the vessels of the vascular area. By far the greater pro- 

 portion of the secretion, no doubt, passes directly through the 

 thin ectoderm of the vascular omphalopleure into the yolk- 

 sac vessels. 



Stage D. — P. obesula. 



Upon the examination of this stage was based the prelimi- 

 nary account already published (1). It is specially important 

 since it shows the allantoic placenta well developed. Both 

 uteri were greatly enlarged ; the left contained two embryos, 

 measuring respectively 8 and 8*25 mm., while the right con- 

 tained a single embryo measuring 8'75 mm. from crown to 

 rump. For the structural characteristics of this latter embryo 

 see table (appendix). 



These three embryos all present substantially the same 

 features of placental connection. Fig. 13 represents the larger 

 of the three embryos attached to the placental area of the 

 uterine wall, and still partially enclosed in its membranes. 

 The dissection from which this figure was made was prepared 

 by opening up the uterus by a ventral longitudinal incision, 

 which also involved the closely adherent oraphalopleural wall. 

 In the figure, then, we see the inner surface of the yolk-sac 

 wall lying on the inner surface of the uterus, which has been 

 spread out flat. 



In the middle of the figure lies the embryo seen through 

 the amnion and the yolk-sac splanchnopleure (y. spl.). At the 

 back of the embryo and partly concealed by the body of the 

 latter is the allantoic placental area {pi. a.), also seen through 



