110 MAMMARY APPARATUS OF THE MAMMALIA 



of the mammary apparatus, from which 

 the later milk organs spring, are homologous — 

 that is, the primary-prim ordia of the Mono- 

 tremes and Marsupials, and the milk-streaks 

 of the Placentals. In the milk-streaks of the 

 Placentalia, therefore, we have preserved a 

 last reminiscence of the primitive brooding 

 organs, which formed the basis of the develop- 

 ment of the mammary apparatus. 



If, then, the milk-streaks are homologous 

 with the primary -primordia of the Marsupials, it 

 is not astonishing that the milk organs derived 

 from them should show essentially the same 

 development in the two orders. Accordingly 

 we find in both, nipples and milk glands so 

 similarly constructed, that — even without any 

 knowledge of the original primordium common 

 to both — one would have assumed that geneti- 

 cally they were nearly related. As a matter 

 of fact, the older authors all agreed in regard- 

 ing the milk organs of the Marsupials and 

 Placentals as identical formations. 



But when Gegenbaur and his school took up 

 the investigation of this subject, especially the 

 study of the relations of the nipples in the 

 different orders of Placentals, they came to 



