Image from page 30 of “The Cambridge natural history” (1895-1909)
– Title: The Cambridge natural history
Year: 1895-1909 (1890s)
Authors: Harmer, S. F. (Sidney Frederic), Sir, 1862-; Shipley, A. E. (Arthur Everett), Sir, 1861-1927
Publisher: London : Macmillan and Co. ; New York : The Macmillan Company
Contributing Library: Earth Sciences – University of Toronto
Digitizing Sponsor: MSN
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Text Appearing Before Image:
POUCH OF MARSUPIALS 17 secondaiy. If this fact be fairly to be interpreted in the sense which I)r. 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 Monotrenies there is a pouch which facilitates or perforins both nutritive and protective functions; in the Phalanger these two functions are carried on in separate pouches ; finally, in other ^Marsupials, there is a return to the undifferentiated state of affliirs found in the Monotremata, but with the help of a new organ not found in them. Though so character- istic 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 Polypro- todont Marsupials ; but in Fhascologale it is only pre- sent in rudiment, and in Miji’jnecohius it is entirely obsolete. In the American Opossums the state of tlie pouch is variable. " Gener- ally absent, sometimes merely composed of two lateral folds of skin separ- ate at each end, rarely complete," is Mr. Thomas’ summary in his definition of the family Didelphyidae.-^ 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 ^ Catalocjue of 3Iarsupials in British Museum, 1886. VOL. X C
Text Appearing After Image:
Fig. 4.—Diagram of the development of the nipple (in vertical section). A, Inditi’erent stage, gland- ular area fiat; B, elevation of the glandular area with the nipple ; C, elevation of the periphery of the glandular area into the false teat, a, PeriiDhery of the glandular area ; b, glandular area ; gl, glands. (From Gegeubaur.)
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