From Xenopus laevis tadpoles which possessed conical hind-limb buds with pigmented melanophores the caudal two-thirds of the tail was amputated. One group of these animals was kept in tap-water and another group in 0.01% phenyl-thiourea in tapwater while regeneration of the tails took place. After one month the tails of the operated animals together with tails from a control group of normal animals at the same stage of development were examined histologically. In the normal tail melanin was only absent from the region of the ventral fin immediately behind the anus: in the untreated regenerate melanin was only absent from the major part of the ventral fin: in the treated regenerate melanin was absent from the whole regenerate. Branched DOPA-positive cells situated beneath the epidermis and each accommodated by a circle 0.05 mm in diameter were present in the normal tails, untreated regenerates, and treated regenerates. They were present only inthe regions lacking melanophores in the normal tail and untreated regenerate, but in the animals treated with phenylthiourea they were present in the unpigmented regenerate and in the pigmented part of the tail-stump. The pigmented melanophores were DOPA-negative. Cells which were stained by the hexamine-silver technique and Schmorl's reaction were found only in the regenerate of the treated tadpoles. It is suggested that the DOPA-positive cells are potential melanophores. In the normal tail and untreated regenerate tyrosinase activity was present without promelanin granules and in the treated regenerate tyrosinase activity was present with promelanin granules. The negative response of pigmented melanophores could reflect a loss of tyrosinase activity as pigmentation of the granules is completed. Phenyl-thiourea does not inhibit melanogenesis in vivo by acting on a metabolic process essential for the normal activity of the cell, or by inhibiting the production and action of tyrosinase, or by preventing formation of promelanin granules, as these were all present in the treated animals. A test-tube experiment showed that phenyl-thiourea inhibits the autoxidation of DOPA. It is suggested that the drug acts by combining with an intermediate produced during the conversion of tyrosine to melanin.

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