100 MAMMARY APPARATUS OP THK MAMMALIA 



to the inguinal region (Fig. 37a). Ver}' soon 

 there appear along this milk-line a series of 

 small swellings (" Milchhiigel "), the first pri- 

 mordia of the individual nipples (Fig. 37b). 

 These next become completely separated from 

 each other by the gradual disappearance of the 

 connecting parts of the milk-line (Fig. 37c). 

 At the same time the nipple primordia become 

 transformed into lens-shaped thickenings pro- 

 jecting into the cutis, and these finally assume 

 the characteristic knob-shaped form. 



Milk-lines not only occur where the nipples 

 in the adult extend over the whole length of 

 the trunk, but also where they are limited to a 

 restricted region. Accordingly the conclusion 

 is justified that originally the Placentalia pos- 

 sessed an elongated row of nipples on either 

 side, and that all deviations from this arrange- 

 ment are due to processes of reduction. Thus 

 the accessory milk organs could be exjjlained 

 as atavistic structures inherited from ancestors 

 provided with a greater number of nipples 

 and milk glands. This explanation of the 

 phenomena of hyperthely and hypermast}^ has 

 completely displaced those older views, ac- 

 cording to which the accessory nipples were 



