Marsupials, there is a return to the undifferentiated state of affairs found in the Monotremata, but with the help of a new organ not found in them. [Illustration] FIG. 4.--Diagram of the development of the nipple (in vertical section). A, Indifferent stage, glandular area flat; B, elevation of the glandular area with the nipple; C, elevation of the periphery of the glandular area into the false teat, _a_, Periphery of the glandular area; _b_, glandular area; _gl_, glands. (From Gegenbaur.) Though so characteristic of Marsupials, the marsupial pouch is not always developed in them. It is present in all the Kangaroos, Wallabies, and Wombats, in fact in the Diprotodonts. It is also present in a number of the carnivorous Polyprotodont Marsupials; but in _Phascologale_ it is only present in rudiment, and in _Myrmecobius_ it is entirely obsolete. In the American Opossums the state of the pouch is variable. "Generally absent, sometimes merely composed of two lateral folds of skin separate at each end, rarely complete," is Mr. Thomas' summary in his definition of the family Didelphyidae.[10] Another curious feature of the pouch in the Marsupials is the variability in the position of the mouth of the pouch: in all the Diprotodonts it looks forward; but in many Polyprotodonts it looks backward. This, however, has some connexion with the habitual attitude of the possessor: in the Kangaroo, leaping along on its hind-legs, it is requisite that the pouch should open forwards; but in the dog-like Thylacine, going on all fours, the fact that the pouch {18} opens backwards is less disadvantageous to the contained young. The male Thylacine has a pouch which is quite or very nearly as well formed as in the female. There are also rudiments of a pouch in the male foetuses of many Marsupials, especially of those belonging to the Polyprotodont section of the order, though these rudiments are by no means confined to that subdivision. Up to so late a period as the age of four months (length 19.8 cm.) the male _Dasyurus ursinus_ has a pouch.