On the phylogeny of hair

The phylogenetic origin of the hair of mammals has puzzled compara-tive anatomists for a century. Many structures of lower vertebrates have been suggested as ancestral to the hair. Among them were certain tactile orgalns of fishes, amphibians, and reptiles; reptilian scales and birds' feathers were also suggested. Reviews of these opinions have been presented by Oppenheimer (1895), Botezat (1914) and Pinkus (1927). During a survey of elevations in the amplhibian skin by Elias and Sha-piro (1957), certain stnrctures were found that resemble hairs. These re-cently found struct:ures, when related appropriately to those previously de-scribed, seem to fill the remainin-g gap in our uniiderstanding of evolution of the scale of the Amniota and of the hair. The feather, most probably a descendant of the scale, constitutes a phy-logenetic side branch. It is not considered in the present discussion. Let us first define the terms "scale " and "lhair." The scale of the Amniota (fig. 1) is a fliat, slanting projection of the corium above the general level of the skin, covered by epidermis. The hair (fig. 2) consists of a downward thickening of the epidermis into the lower end of which a corium papilla projects. From this down-ward thickening an extremely long and acute cone of epidermis projects above the general level of the skin; this projecting cone is surrounded by a ditch. The organ is innervated anid can serve for the perception of pres-sure, but serves particularly for the perception of lateral displacement of its freely projecting part. Thus the mammalian hair functions as a sen