Undifferentiated genital ridges of Bufo bufo tadpoles, after 26 days of organotypic culture, are unable to differentiate either Bidder organ or gonadic part. Genital organs of metamorphosing animals, after 20 days of culture, preserve ripening oocytes in Bidder organ; in the gonad some gonocytes in mitosis are present, but no sex-differentiation occurs. Differentiated genital organs of new-metamorphosed animals maintain quite normal Bidder organs until 30 days of cultivation; in the gonads how ever after 20 days conspicuous alterations occur, particularly consisting in a strong decreasing in the number of gonocytes.