ABSTRACT
This investigation was made in collaboration with Sir Edward Mellanby, F.R.S. (National Institute for Medical Research, Mill Hill). It has long been known that in animals suffering from vitamin A deficiency, mucous epithelia are replaced by keratinizing epithelium of the epidermoid type. Since keratinization is promoted by lack of vitamin A, it seemed possible that it might be inhibited by an excess of the vitamin. Series of cultures were therefore made in which explants of the ectoderm and underlying tissue from the trunk and limbs of 6–7-day chick embryos were grown by the watchglass or hanging-drop methods; the explants from one side of each embryo were grown in normal medium, and corresponding fragments from the opposite side in medium to which about 1,800 i.u. per 100 c.c. of vitamin A had been added ( + A medium).
