Marsupial placentation and its evolutionary significance.

Cumulative evidence supports the premise that the bandicoot and all other marsupials are truly placental mammals. The total reliance of most marsupials on the yolk sac placenta provides a clean-cut opportunity to define its functional activity, since there is no overlap with chorioallantoic function as in eutherians. The unique appearance of a marsupial chorioallantoic placenta in the family of bandicoots is a remarkable evolutionary imprint that merits further consideration. In late gestation, the chorioallantoic trophoblast disappears as a layer; available evidence suggests that it may have fused with maternal homokaryons to create heterokaryons only at this site. It is possible that, in the bandicoots, only the initial phases of implantation occur, i.e. cell attachment and cell fusion. The bandicoot placental heterokaryons may represent the survival of an early stage in the evolution of the mammalian chorioallantoic placenta.