THE INTEGUMENT - AND THE EXOSKELETON 87 



will be noticed that scattered hairs appear among the scales 

 in a definite relationship, and that a group of three hairs, one 

 median and two lateral, projects from beneath the margin of 

 each scale, the median hair being somewhat longer and stouter 

 than the others. It further appears that there is a similar ar- 

 rangement of hairs, usually in groups of three, upon hair areas 

 not associated with scales, the hair groups 'being arranged in 

 imbricated series, and that this arrangement is general, even in 

 mammals without trace of scales. There are some modifica- 



a 







G 



FIG. 21. Hair arrangement 

 in various mammals. 



(a) Ursus arctos (brown 

 bear). Front of chest. Dia- 

 . grammatic. [After DE MEI- 

 JERE.] (b) Canis familiaris 

 (dog). Four developmental 

 stages. The adult arrange- 

 ment is like that of (a). 

 [After DE MEIJERE.] (c) 

 Homo. Scalp of negro. Cam- 

 era drawing from the actual 

 object. 



tions of this, due to secondary changes, such as the need of a 

 thick fur, but even in these modifications the original plan 

 is still apparent. Thus, in the pig, there are two sets of bris- 

 tles, a coarser set arranged in imbricated groups of three, and 

 a finer set, filling the intervening areas without definite ar- 

 rangement; to obtain a thick fur, as in the rabbit, each hair 

 in the group may become a bundle of hairs, the bundles being 

 arranged in groups of three as in the typical case; even the 

 number three is not always kepi, and groups of five occur, 



