Abstract
These experiments describe further investigations into the effects of vitamin A on regenerating limbs. The effects of different retinoids, the time of administration, concentration of vitamin A and histological, autoradiographic and histochemical studies are reported. The most obvious result of vitamin A treatment is to cause proximal elements to regenerate from distal amputation levels, that is to cause serial reduplication of pattern in the proximodis-tal axis. Retinoic acid was the most potent of the analogues tested and longer times of administration or higher concentrations cause a greater amount of serial reduplication. Various tissue changes have been found which include the inhibition of cell division, loss of cartilage metachromasia, changes in the mucous-secreting properties of the epidermis and an increased packing in the blastemal cells. The significance of these cellular effects in relation to the pattern-formation changes is discussed.