Placenta of the gray four-eyed opossum

The gray four-eyed opossum (Philander opossum) by André de Souza Pereira
Wikipedia Commons CC BY-SA 3.0

The gray four-eyed opossum (Philander opossum) is widely distributed in South and Central America. Its placentation has been described from animals caught on Barro Colorado Island in Panama (here).

Yolk sac of Philander opossum. The sinus terminalis marks the transition
between the avascular and vascular portions.
Courtesy of Dr. Allen C. Enders

As in the Virginia opossum, the yolk sac is comprised of avascular and vascular portions. The avascular part follows the contours of the uterine epithelium (at left in the figure). The vascular portion attaches to it (at right).

Semi-thin section of vascular yolk sac (at top) and fold in the uterine
wall (below) in Philander opossum.
Courtesy of Dr. Allen C. Enders

The study of Philander by Enders and Enders was the first to show invasive trophoblast in a marsupial. In the figure there is a fold of uterine wall with intact epithelium at left and right, but this has disappeared at the centre of the fold, which is occupied by large trophoblast cells. Subsequently invasive trophoblast has been described in another opossum (Monodelphis domestica) and in an Australian marsupial (Sminthopsis crassicaudata).

Photomontage of a section through one uterus of Philander opossum.
The two fetuses share a common yolk sac, but each has its own
allantoic sac one of which can be seen.
Courtesy of Dr. Allen C. Enders

An unusual feature in Philander is that two or more fetuses share a common yolk sac. However, each has its own allantoic sac. The allantois never makes contact with the trophoblast (it remains within the small exocoelom). It may serve as a receptacle for urine secreted by the mesonephros.