II POUCH OF MARSUPIALS ] 
Py 
‘ 
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 
undifferentiated 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 
Phascologale 1t is only pre- 
sent in rudiment, and in 
Myrmecobius it is entirely 
obsolete. In the American 
Fria. 4.—Diagram of the development of the nipple 
Opossums the state of the (in vertical section). A, Indifferent stage, gland- 
pouch is variable. “Gener- ular area flat ; B, elevation of the glandular oe 
: with the nipple ; C, elevation of the periphery 
ally absen t, sometimes of the glandular area into the false teat. a, 
Periphery of the glandular area; 6, glandular 
merely, composed of two area; gl, glands. (From Gegenbaur.) 
lateral folds of skin separ- 
ate at each end, rarely complete,” is Mr. Thomas’ summary in his 
definition of the family Didelphyidae.t 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 Cc 
