Four sets of experiments were performed in which developing Xenopus laevis were treated with 0.1% phenylthiourea. The animals used in experiments 1 and 2 were taken from a single batch of eggs, those in experiment 3 from another batch of eggs, and those in experiment 4 from a third batch. A suitable control group of untreated tadpoles was arranged for each experiment. In experiment 1 gastrulae were reared in a solution of phenylthiourea, which inhibited melanogenesis. In experiment 2 tadpoles, which possessed melanophores containing melanin, were kept in a solution of phenylthiourea for 6 weeks. No turnover of melanin was detected in these animals. Experiment 3 was carried out on tail regenerates. It showed that melanogenesis is inhibited by phenylthiourea and that no turnover of melanin occurs during treatment with this drug. In experiment 4 animals which had almost completed metamorphosis showed no evidence of turnover of melanin when treated with phenylthiourea.

The arrest of development of the tadpoles and the histological changes in the thyroid glands which occurred during treatment were used as evidence that phenylthiourea reached the tissues of the experimental animals.

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