46 MAMMARY AITARATUS OF THK MAMMALIA 



fications are considerable, especially among 

 the latter, as can i-eadily be seen from a 

 study of the table. It is important to re- 

 member that in certain forms, not a complete 

 pouch, but onl}' a pair of lateral folds may 

 exist, and also that in a great number of 

 species the marsupium is missing altogether. 



Inside the marsupium are the milk organs, 

 which, unlike those of the Monotremes, have 

 been so perfected that the efferent ducts of 

 the milk glands do not end on a flat gland 

 area, but in a nipple. The nipples of the 

 Marsupialia, however, are distinguished from 

 those of the Placentalia by a very striking 

 featui-e, the discovery of which we owe to 

 the famous researches of John Morgan in 

 1833. Morgan stated that the marsupium 

 of the virgin kangaroo does not contain any 

 nipples, but that there are instead in the 

 region in question four small circular aper- 

 tures, each one leading into a channel at 

 the bottom of which on a small papilla 

 the ett'erent ducts of the milk glands open 

 (Fig. 15a). This condition remains the same 

 for a long time. Only at the beginning of 

 pregnancy, or shortly before lactation com- 



