supials.[1] 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.[2] 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.[3] 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)
- ↑ See Haacke, "On the Marsupial Ovum, the Mammary Pouch, etc., of the Echidna," Proc. Roy. Soc. 1885, p. 72; and "Über die Entstehung der Säugetiere," Biol. Centralbl. viii. 1889, p. 8.
- ↑ See Gegenbaur's Elements of Comp. Anat. Transl. by Bell, 1878, p. 421.
- ↑ "Über die Beziehungen zwischen Mammartasche u. Marsupium," Morph. Jahrb. xvii. 1891, p. 483.
