THE INTEGUMENT AND THE EXOSKELETON 97 



not before them, and may have exercised some function of 

 protection, possibly that of a fringe upon or near the free 

 edges to prevent the accumulation of dirt in the folds where 

 they overlap, a purpose for which organs of similar appear- 

 ance but of different origin are frequently employed in in- 

 sects and crustaceans. 



In development the hair is wholly epidermic, formed by 

 the stratum germinativum, but dips down into the corium 

 in the form of a solid column of rapidly proliferating cells, 

 the outer layer of which soon differentiates into a sheath or 

 follicle, while the inner cells become horny and form a shaft 

 which projects beyond the surface and becomes the hair. 

 Growth is constantly kept up at the bottom of the follicle, 

 and proceeds from a small area of actively proliferating cells 

 which are nourished by a corium papilla and form the true 

 root, or matrix, of the hair. From an inspection of the fol- 

 lowing figure (Fig. 26), it becomes evident that this matrix 

 is merely a specialized portion of the stratum germinativum 

 and that the hair consists of the upper layers derived from it, 

 and renewed from beneath as in the superficial skin. When 

 a hair is pulled out, the break usually occurs immediately above 

 the matrix, and the lost portion involves the hair, the epider- 

 mic sheath, and quite often the follicular sheath as well, parts 

 that are easily regenerated so long as the matrix remains. 

 Associated with this structure are typically two sorts of glands, 

 tubular and acinous, which are formed as outpushings from 

 the sides of the follicle and grow down into the corium. These 

 develop in various mammals to subserve many different pur- 

 poses, often becoming dissociated from the original connec- 

 tion with the hair. To these two types all forms of integu- 

 mental glands occurring in mammals may be referred. Their 

 modifications and transformations may be considered later. 



The occurrence and distribution of the hair are. in strict ac- 

 cordance with the needs of the animal, and show great dif- 

 ferences, corresponding to the various environments to which 

 mammals have become adapted. The hair may differ in 

 length, in caliber, in thickness (/. e., the number of groups in 



