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The frog provides among the vertebrates the best opportunity to investigate the factors which determine loss of regeneration within the ontogenetic boundaries of a single organism, for in tadpoles the capacity to regenerate tapers off along the proximo-distal axis of their limbs and in adult frogs limb regeneration is substantially absent. It was generally accepted that metamorphosed frogs lose this capacity because of progressive complexity of structure in the growing limbs (Marcucci, 1916; Polejaiev, 1936; and Forsyth, 1946). However, the conclusion that it is the nature of the limb tissues which determines presence or absence of regeneration was shown to have only limited validity when regeneration was obtained in metamorphosed frogs. Thus Polezhajev (1945) demonstrated that repeated trauma to the amputational surface of limbs of adult frogs (which according to his own theory had irrevocably lost the powers of regeneration because of progressive differentiation) was sufficient for recuperation of this faculty in a substantial number of cases.

Effects of Adrenal Transplants upon Forelimb Regeneration in Normal and in Hypophysectomized Adult Frogs