The Birth-period of Trichosurus vulpecula. 



95 



or mammary pockets, out of which it is supposed to have taken its 

 first origin. Since the development, as far as the formation of mam- 

 mary pockets, is very similar in both Metatheria and Eutheria, i. e. 

 as far as a stage antecedent to the formation of a pouch, there is no 

 escape from the conclusion drawn above. The mammary line of 

 ScHULTZE has probably no greater significance than any other deve- 

 lopmental structure, which, first appearing as a distinct line or ridge, 

 afterwards becomes broken up into a"^'number of separate entities. 



It appears, therefore, to be clear that the mammary apparatus 

 of such a typical Eutherian mammal as the pig, in which it is better 

 developed than elsewhere ") among the Eutheria, remains in reality at 

 a stage of development lower than in a typical marsupial, or in other 

 words, that the mammary apparatus of Eutheria is, as 

 compared with that of Metatheria, arrested in its de- 

 velopment. 



1) Except in Centetes. 



newly-born Trichosurus vulpecula. 



