106 MAMMARY APPARATUS OF THE MAMMALIA 



The difficulty here indicated directs our 

 attention to a fact which, although known 

 more than fifteen years, has so far been 

 entirely left out of account in the discussions 

 as to the significance of the milk-line. And 

 that is the fact that the milk-lines in no wise 

 represent the first primordia of the mammary 

 apparatus of the Placentals, but, as numerous 

 investigators have shown, are differentiated 

 secondarily from other formations, usually 

 called " milk-streaks " ( Fig. 38 ). These milk- 

 streaks appear in early embryos in the region 

 of the later milk-lines as two comparatively 

 broad zones of the integument, characterized 

 by higher epithelium and by a condensation of 

 the mesenchyme cells of the cutis, penetrated 

 by numerous capillaries. It is not till later 

 that the central region of each streak becomes 

 elevated to form the milk-line, while the rest 

 of it disappears. This being ascertained, two 

 conclusions follow : First, it follows that those 

 investigators were in the wrong who, like 

 Beard, Profe, and others, compared the milk- 

 lines with formations such as the neural crest 

 and the dental lamina, w^hich actually repre- 

 sent nothing but developmental structures, and 



