In the Monotremata (in _Echidna_) there is a deep fold of the skin which lodges the unhatched egg, and into which the mammary glands open, one on either side. This structure is only periodically developed, and arises from two rudiments, one corresponding to each mammary area; but in the female with eggs or young there is but a single deep depression, which occupies the same region of the body as the marsupial pouch of the {16} Marsupials.[7] It is usually held that this structure is not of precisely the same morphological value as the pouch of the Marsupial; and the difference is expressed by terming the one (that of _Echidna_) the mammary pouch, and the other the marsupium. At first sight it may appear to be an unnecessary refinement to separate two structures which have so many and such obvious likenesses. It is not quite certain, however, that the difference is not even more profound than later opinions seem to indicate. The Monotremata not only have no teats, as has already been pointed out, but the mammary glands themselves are of a perfectly different nature to those of the higher mammals, including the Marsupials. There is therefore no _a priori_ objection to the view that the accessory parts developed in connexion with the mammary glands should also be different. The teat of the higher Mammalia grows up round the area upon which the ducts of the mammary glands open; it is a fold of skin which eventually assumes the cylindrical form of the adult teat, and which includes the ducts of the milk glands. It has been suggested that the two folds of skin which form the mammary pouch of _Echidna_ are to be looked upon as the equivalent of the commencing teat of the higher mammal.[8] In this case it is clear that the marsupial folds of the Marsupial cannot correspond accurately with the apparently similar folds of _Echidna_, because there are teats as well. It is the teats which correspond to the marsupial folds of _Echidna_. This view is in apparent contradiction to an interesting discovery in a specimen of a Phalanger by Dr. Klaatsch.[9] This Marsupial, like most others, has a well-developed marsupial pouch, in which the young are lodged at birth; but round two of the teats is another distinct fold on either side, the outer wall of which forms the general wall of the pouch. Dr. Klaatsch thinks that these smaller and included pouches are the equivalents of the mammary pouches of _Echidna_. They contain teats, but this comparison does not do away with the validity of Gegenbaur's suggestion already referred to, because the teats are (see above) {17} secondary. If this fact be fairly to be interpreted in the sense which Dr. Klaatsch attaches to it, we have an interesting case of the growth of a new organ out of and partly replacing an old organ. In the Monotremes there is a pouch which facilitates or performs both nutritive and protective functions; in the Phalanger these two functions are carried on in separate pouches; finally, in other