138 MAMMARY APPARATUS OF THE MAMMALIA 



view as incorrect, which would see in the 

 Marsupials specialized descendants of Placental 

 ancestors. No matter what parts of the 

 mammary apparatus we consider, we find 

 always that the condition in the Placentalia 

 never throws any light on that of the Mar- 

 supialia, but that, on the contrary, the reverse 

 is regularly the case. As we have seen, milk- 

 streaks and milk-lines were unintelligible until 

 the discovery of the primary-primordia of the 

 Marsupials. Again, the vestigial nipple pouches 

 which occur in the Placentalia only during 

 embryonic development could not be explained 

 without the knowledge of the nipple pouches 

 of the Marsupials. The same holds true of 

 the mammary hairs. Thus the course of the 

 phylogenesis cannot possibly have led from a 

 Placental stage to the Marsupials. 



On the other hand, all our observations bear 

 witness to the truth of Huxley's view. I would 

 emphasize that it is only at the commencement 

 of the development of the mammary apparatus 

 in the Monotremes and Marsupials that we 

 meet with identical formations — namelv, the 

 primary-primordia. All the other structures of 

 the mammary apparatus must have developed 



