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. [Illustration: FIG. 114.--Oeba Armadillo. Photo: Berridge.] [Illustration: FIG. 115.--Sea-cows.] We shall now ask the reader to conceive a primitive placental mammal type, and to consider briefly the relationships of the various groups that have sprung from it. The group whose evolution has been perhaps least progressive is that of the Edentates, including the sloths, armadillos, and American ant-eaters (Fig. 114). Then, on the one hand, we have the herbivorous types, including the remarkable sea-cows (Fig. 115), the rodents (Fig. 116), and the hoofed animals, leading up from such comparatively primitive forms as the coneys (Fig. 117) to such highly specialised types as the giraffe (Fig. 118). The other series of groups are believed to be more nearly related to each other than to the aforementioned, and the nearest approach to their common ancestor, among modern mammals, is probably to be found in the Insectivora (Figs. 119 and 120). Differing sharply from the Insectivora in their possession of wings, but otherwise closely similar, are the bats (Fig. 121). From this same insectivor-like type have probably evolved the whales (Fig. 122), the seals, etc. (Fig. 123), the carnivora (Fig. 124), and finally the primates, leading up through the lemurs to the monkeys and man. Thus merely indicating the many and devious roads of mammalian Evolution, we turn again to the one particular line that we set out to follow. [Illustration: FIG. 116.--Porcupine.] [Illustration: FIG. 117.--Coney (Hyrax).] [Illustration: FIG. 118.--Giraffe. Photo by _Sport General_.] [Illustration: FIG. 119.--_Centetes ecaudatus_, a primitive insectivor.] [Illustration: FIG. 120.--Hedgehog. Photo: Berridge.]