Fig. 112.—Marsupials—Long-nosed Bandicoot.
Fig. 113.—Squirrel-like marsupial (Phascologale penicillata).
As has previously been indicated, the most important characteristic of the third great group is a modification of the membranes of the embryo to form a connection between it and the wall of the uterus. The allantois develops as a highly vascular membrane, the small blood vessels of which are brought into very close contact with those in the wall of the uterus. So that, while the blood of the mother does not actually mix with that of the child, the two fluids are separated only by thin membranes, through which nutritive substances easily pass. The broad advantage of this is, of course, that the young animal passes the earlier stages of its life inside the mother's body, where it is exposed to a minimum of risk, is efficiently nourished, and from which it is not sent forth into the world until it is tolerably well able to look after itself.
Fig. 114.—Oeba Armadillo.
Photo: Berridge.
Fig. 115.—Sea-cows.