IN MONOTEEMES AND MARSUPIALS 87 



that the Monotremes have at most one or two 

 young ones, but the most primitive Marsupials, 

 on the other hand, a much greater number, ten 

 and more. 



In the Monotremes, therefore, the two gland 

 areas sufficiently provide for the needs of the 

 one or two young. But in forms producing a 

 much larger number of offspring, such a simple 

 arrangement of mammary organs was obviously 

 not satisfactory. Accordingly we find in the 

 Marsupialia that the primordia of the mammary 

 apparatus, given in the original brooding 

 organs, take another developmental course : (1) 

 the area of the primary-primordia has become 

 divided vip into several separate parts (the 

 nipple primordia), corresponding approximately 

 to the number of the young ; (2) the surface 

 of these separate primordia has become in- 

 creased by way of invagination. 



This last indeed happens very frequently in 

 gland- bearing regions of the integument in 

 numerous classes of the animal kingdom, 

 invertebrate as well as vertebrate. Sometimes 

 these glandular invaginations can be everted, 

 as is the case, for example, with the so-called 

 dorsal gland of the Peccary [Dicotyles 



