CHAPTER II 



WE pass on next to consider the de- 

 velopment of the mammary organs of 

 the Didelphia. Bat first of all some account 

 of their structure in the adult. 



As the name Marsupialia, often used for 

 Didelphia, indicates, their mammary appar- 

 atus is distinguished especially by the exist- 

 ence of a marsupium, usually a capacious 

 pouch, surrounded by a strong sphincter 

 muscle, the sphincter marsupii, and en- 

 closing the milk organs. With the exception 

 of the external opening of the pouch, some- 

 times directed straight down, sometimes 

 towards the head or towards the tail, the 

 marsupium appears similarly formed in most 

 families of the Didelphia, a list of which is 

 given in the table. Greater differences in its 

 structural conditions occur onl}- amongst the 

 lowest families, the Australian Dasyuridte 

 and the American Didelphyidaj. The modi- 



44 



