POUCH OF MARSUPIALS 



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 Mouotremes 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 

 undifferentiated state of affairs found in the ]\Ionotreniata, but 

 with the help of a new 

 organ not found in them. a ^ 



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 

 Phascologale it is only pre- 

 sent in rudiment, and in 

 Myrmecoljius 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.^ 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 haltitual 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 



^ Catalogue uf Marsiqncds in British Museum, 1886. 

 VOL. X d 



Fig. 4. — Diagram of the development of the nipple 

 (in vertical section). A, ludilferent 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 ; h, glandular 

 area ; gl, glands. (From Gegenbaiir.) 



