|Embryo of Virginia Opossum (Didelphys virginiana) at 6 days |
of gestation. Opened to show the vessels of the trilaminar yolk sac
and free-floating allantoic sac. From Selenka 1886.
All marsupials have a yolk sac placenta with both two-layered and three-layered areas (bilaminar and trilaminar omphalopleure); the latter has blood vessels that radiate from a sinus terminalis as can be seen in Selenka's illustration.
|Neonate of Virginia Opossum. From Selenka 1886.|
Note that the allantois makes no contact with the trophoblast. Unlike in the koala, wombat and bandicoots, there is nothing approaching chorioallantoic placentation. The allantois serves mainly as a receptacle for urine excreted by the mesonephros (here).
|Virginia Opossum (D. virginiana) by Cody Pope CC BY-SA 2.5|
Despite the species richness, placentation has been described for only five opossums; three from the same genus (D. virginiana, D. aurita and D. marsupialis) plus the gray short-tailed opossum (Monodelphis domestica) and gray four-eyed opossum (Philander opossum). I will save them for a later post.