Tadpoles of a sexually undifferentiated race of Rana latastei were injected with a testosterone oil-solution. After six weeks a group of testosterone treated animals was injected with an actinomycin D water solution. Finally, after eight weeks, treated and control animals were killed a day after the injection of a 3H-uridine water-solution. In control specimens the gonads preserved their typical ovarian structure; in testosterone treated specimens a sex reversal was constantly induced; in specimens treated with testosterone plus actinomycin D the medullo-stimulating effect of testosterone was suppressed. By means of autoradiographic studies after 3H-uridine incorporation the testosterone target-cells have been identified in the extragonadic interrenal region; firstly these activated cells strongly increase the RNA-neosinthesis, and successively they begin to descend towards the ovaries along their mesogonia, so operating their sex reversal. These two steps of the testosterone-induced masculinization were disconnected by the pulse administration of actinomycin D.
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