NIPPLE DEVELOPMENT OF PLACENTALS 115 



^"^ifi^ccui 







Fig. 40. — Nipple Development of Placentals. 



X, Knob-shapecl primordia common to Marsupials and Plaeentals. 



never the case in the Placentalia. Here an in- 

 volution of the knob-shaped primordium begins, 

 which is marked by the appearance of a shallow 



