POUCH OF MARSUPIALS 



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 Marsupials, there is a return to the 

 ^differentiated state of affairs 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 

 Pliascologale it is only pre- 

 sent in rudiment, and in 

 Myrmecobius it is entirely 

 obsolete. In the American 

 Opossums the state of the 

 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. 1 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 



1 Catalogue of Marsupials in British Museum, 1886. 

 VOL. X C 



FIG. 4. Diagram of the development of the nipple 

 (in vertical sectioii). A, Indifferent stage, gland- 

 ular 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 t glands. (From Gegenbaur.) 



