Keratin production by the epidermis is a relatively recent phylogenetic development. The integument of invertebrates, which corresponds to the epidermis of vertebrate species, contains complex mucopolysaccharides. Of these, chitin is the most important and the most widespread among the various phyla.1-3 This substance, polyacetyl glucosamine, is combined with protein, usually in association with inorganic salts, such as calcium carbonate, which form the supportive exoskeleton. In vertebrates, chitinous cuticles are entirely absent. In fish, in addition, a highly developed keratinization process is in operation; mucoproteins are produced in the epidermis by specialized glands.4 While the presence of mucopolysaccharides in mammalian epidermis has been suspected for a long time,5 it was only during the last decade that significant advances have been made in this field. Ten years ago Wislocki, Fawcett, and Dempsey published a report,6 remarkable for its foresight and potential significance. While studying the epithelia of various