It was for a long time held that the Marsupials differed from all other mammals in having no allantoic placenta.
As the allantois increases in size and importance, the allantoic vessels are correspondingly developed.
The allantoic arteries in the cord wind in a spiral manner round the allantoic vein.
The intra-embryonic portion of the allantoic stalk gives rise to two structures, viz.
The last of these can be dismissed, in consequence of the recent discovery of an allantoic placenta in Perameles.
In a somewhat older fœtus of ten lines in length there was a small allantois supplied by two allantoic arteries and one vein.
The allantoic cavity persists (Owen), but the allantois only covers the placental area of the chorion.
The allantoic sack is large, and covers the whole surface of the subzonal membrane.
Krause maintains that all the other embryos shewing an allantoic stalk at an early age are pathological.
The allantoic stalk, if present, was not clearly made out, and the condition of the amnion was also not fully studied.