NATURE OF THE MILK-LINE 105 



of nipples. In other words, we should expect 

 to find in these forms milk-lines. But this is 

 not the case, as we know. 



We have seen, however, in the foregoing 

 chapter, that there are otlier formations in the 

 Marsupials — namely, the primary-primordia, 

 which represent the common starting-point for 

 the differentiation of the nipples on either side 

 of the body. 



The recognition of this fact reveals to us a 

 noteworthy parallelism between the first pro- 

 cesses in the development of the mammary 

 apparatus of the Marsupials and Placentals, and 

 leads us at once to consider whether the milk- 

 lines of the Placentals do not correspond to the 

 primary - primordia of the Marsupials, and 

 whether such a comparison might not afford 

 the basis for a new explanation of the nature 

 of the milk-lines ? 



This question, however, is not so easily 

 solved. For comparison of the primary- 

 primordia and the milk-lines is confronted by 

 the difficulty that the two formations are so 

 different in appearance. In face of this mor- 

 phological diversity we are not justified in at 

 once homologizing them. 



