122 



CHORDATE ANATOMY 



the human forearm suggests his animal ancestry. The hair of the fore- 

 arm slants from the wrist toward the elbow, in the reverse direction to 

 the slant on the upper arm. Man shares this pecuHarity with the apes 

 alone. All other mammals have the same hair direction on both parts 

 of the limb. Why this resemblance of man to the apes unless they share 



a common ancestry? The pecuharity is 

 not adaptive, and it is not easy to see 

 why, if man and apes were independently 

 created, they should resemble one another 

 in this detail. 



Hair Arrangement. That the 

 arrangement of hairs on the human body 

 has any evolutionary meaning is, to say 

 the least, surprising. Indeed, since such 

 patterns can have no use, we should 

 hardly expect to find them at all. No 

 less surprising is an arrangement of hair 

 in mammals that indicates descent from 

 scaly ancestors. 



In most mammals, the hairs occur in 



Fig. II';. — Arrangement of the , rn, 



hairs in groups of threes and fives groups of three.or more. These groups 

 in the human embryo, with the g^j-g arranged in parallel rows in such wise 

 l^t:^L'"Trot KlSsTe^'aft:' that each cluster Ues opposite an interval 

 Stohr.) in the rows in front and behind. In short, 



the arrangement is imbricated, Hke the universal arrangement of scales. 

 This arrangement, though quite useless, is precisely what we should expect 

 if mammals have descended from scaly ancestors. See Fig. 115. 



Histogenesis of Hairs. Hairs are, in origin, epidermal, and therefore 

 ectodermal. Each begins as a minute epidermal papilla, which has arisen 

 by local cell proliferation in the stratum germinativimi. See Fig. 116. 

 Continued proUferation gradually converts this papilla into a cellular 

 column, which extends obliquely downward into the underlying mesen- 

 chyma which is to become the corium. The growing end swells into a bulb, 

 in which later develops the corium papilla from which the hair is to grow. 

 Cellular differentiation of the hair column results in an inner sheath and the 

 hair-shaft, all surrounded by an outer sheath. From the bulb to the 

 point in the hair column where the sebaceous gland develops, the cells 

 of the hair-shaft become cornified. Above this point the central cells 

 degenerate to form a canal in which the hair-shaft grows towards the 

 surface. Continued cell multiplication of the stratimi germinativum 

 of the papilla elongates the central hair-shaft to extend beyond the skin. 

 Each hair thus formed continues to elongate throughout its life of 

 several months or years, the rate of growth varying greatly in different 



