Fig. 1. Generalized features of integument in four major clades of vertebrates. and ß indicate alpha- and beta-type keratins, respectively. OG and IG indicate outer and inner epidermal generations that might be present in a squamate reptile prior to skin shedding. The mesos layer of squamates is a specialized cell type derived from keratins and contains extracellular, laminated lipids, comprising the principal water barrier in these species.