The embryonic membranes of the diprotodont marsupials Trichosurus vulpecula, Pseudocheirus peregrinus (Phalangeridae) Protemnodon rufogrisea, Setonix brachyurus and Potorous tridactylus (Macropodidae) are described.

Three egg membranes—zona pellucida, albumen layer and shell membrane—are present. The shell membrane persists for almost the entire gestation period (17.5 days) in Trichosurus and for about 20 days in Setonix which has a gestation period of 27 days.

All species have yolk-sac placentation and in none does the allantois take part in the formation of a placenta. The bilaminar omphalopleure (non-vascular yolk-sac placenta) appears early and the vascular omphalopleure comparatively late in uterine development.

The amnion and allantois appear comparatively late in pregnancy. In Setonix amnion folds are not evident at 17 days and are incompletely developed at 19 days. No allantoic primordium may be recognised in Setonix at 19 days The allantois is however of comparatively large size in near term embryos of Pseudocheirus, Protemnodon and Setonix. In Pseudocheirus the allantois reaches almost to the chorion but in Protemnodon and Setonix it is excluded from the chorion by the great development of the yolk-sac splanchnopleure.

The early embryos of all species are orientated with the head towards the anterior end of the uterus but late embryos of Trichosurus, Pseudocheirus, Protemnodon and Setonix have the head towards the posterior end of the uterus. Late embryos of Potorous have not been studied. In all species the bilaminar omphalopleure is apposed to the anterior part of the uterine wall and separated from the posteriorly placed vascular omphalopleure by the sinus terminalis which follows an equatorial or oblique course around the inner surface of the uterine wall. A small area of true chorion is apposed to the uterine wall near the uterine neck.

In all species the uterine lumen epithelium remains complete throughout pregnancy. Embryonic and maternal blood streams are separated by the two endothelia, by embryonic mesenchyme and maternal connective tissue and by embryonic ectoderm and maternal epithelium. The uterine endometrium is well vascularised in Trichosurus and Pseudocheirus and in the latter embryonic and maternal blood streams are brought into close proximity by displacement of nuclei from the cell layers separating the blood vessels. In the remaining species (all macropod marsupials) the uteri are relatively avascular and embryonic and maternal blood streams are well separated.

Material giving a positive periodic acid-Schiff reaction is found in the uterine glands and appears to serve an embryotrophic function. Further P.A.S. positive material, found within the embryonic membranes, gives positive tests for glycogen. In Protemnodon a considerable amount of material, consisting of degenerating cells, apparently serves a histotrophic function. The vascular omphalopleure appears to be the main organ of embryotrophic nutrition in the closing stages of pregnancy.

It is suggested that yolk-sac placentation is the primitive marsupial condition and that allantoic placentation has been evolved, within the group, in at least two separate lines.