MILK-LINES AND MILK-STREAKS 109 



forms where this extension of the nipple rows 

 does not exist, conditions are observable which 

 are very similar to those in the Marsupials, 

 as, for instance, in the Ungulates, whose milk 

 streaks and lines do not reach cranially beyond 

 the region of the navel. On the other hand, 

 it is not impossible that the primary -primordia 

 of those species of Peramys and Marmosa 

 amongst the Marsupials, having long rows of 

 nipples, may be found to be similar, as regards 

 their extension, to the milk- streaks of the 

 Placentalia. 



The importance which hitherto has been 

 attached to the milk-lines in discussions con- 

 cerning the genesis of the mammary apparatus 

 must now accordingly be transferred to the 

 milk-streaks. The milk-lines are merely 

 secondary, ridge -like differentiations within 

 the milk-streaks, for the understanding of 

 which this fact is perhaps of importance — 

 namely, that the disposition to form such 

 ridge-like structures already exists within the 

 primary-primordia of Echidna and the Mar- 

 supials. 



It now becomes clear that in all three orders 

 of Mammals, the first indifferent primordia 



