THE MILK-STREAK 107 



which are not preceded by any primordia cor- 

 responding to the milk-streak. Secondly, it 

 follows that when we trace back the mammary 

 apparatus of the Placentals, we have to com- 

 pare the primary-primordia, not with the milk- 

 lines, but with the milk-streaks. And indeed 

 this comparison presents no difficulty. 



Fig. 38. — Human Embryo, showing "Milk-Streak" 

 AND Central Region Becoming Elevated to 

 Form "Milk- Line." (Schmitt.) 



11 '5 mm. 



It is clear, then, from the above statements 

 that the primary-primordia and the milk-streaks 

 are entirely similar in structure. In both 

 formations we recognize (1) an area of 

 thickened epidermis ; (2) an underlying zone 

 of condensed and richly vascularized cutis. 

 Topographically, also, both harmonize fully. 



