[Spontaneous and induced metamorphosis inXenopus larvae at low temperature].

In premetamorphicXenopus larvae mortality is not influenced by lowering the temperature from 22° to 8.5° C, but it rises dramatically below 8.5° C. At 10° C complete inhibition of spontaneous metamorphosis occurs only in premetamorphic larvae; beyond stage 56 cold treatment only delays the metamorphic changes.In cold-arrested premetamorphic larvae thyroxine and TSH elicit metamorphic responses. Since the responding capacity of the larval tissues to thyroxine is not affected, blockage of spontaneous metamorphosis at low temperature must result from the inhibition of the hypothalamic center.