. Chordate anatomy. Chordata; Anatomy, Comparative. 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. H
. Chordate anatomy. Chordata; Anatomy, Comparative. 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 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 differentiati
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