While up until the 1870s the skin covering of vertebrates had aroused only little interest, it has—especially in very recent times—become the subject of most diligent study. The most contradictory opinions have been voiced, and particularly fierce at this very moment is the battle that has erupted over the question of whether mammalian hairs are homologues of reptilian scales or not. This work, however, will strictly confine itself to scale formation and phylogenesis, and is intended only to introduce a series of contributions to the morphogenesis of the integument.