294 THE AMERICAN NATURALIST [ VOL. LIY 



along with the activities of the ostrich there appears no 

 reason why the sternal, pubic and alar callosities should 

 not be regarded as direct, structural responses to the 

 pressure and friction to which these parts of the body are 

 subject in the e very-day activities of the bird. They 

 could be understood as acquired, adaptive characters. 

 The experiment given has served to prove, what would 

 naturally be expected from experience with other ani- 

 mals, that the skin generally is endowed with the power 



to make callous responses when subjected to the neces- 

 sary stimuli. 



It was with some surprise therefore that in a series of 

 embryos, representing all the stages passed through dur- 

 ing the 42 days of incubation of the ostrich, the later ones 

 were found to possess a perfectly developed callosity over 

 both the sternum and the pubis, of exactly the same form 

 and nature as in the young chick and adult (Fig. 4). The 

 papillary outlines shown to be such a characteristic fea- 

 ture of sauropsidan callosities have the same variations 

 in size and distribution as in the adult, and serve clearly 

 to delimit the callous area from the remaining smooth 



