In Drosophila the genital ducts arise from the so-called genital imaginal disk which in the larva lies at some distance from the gonads. In the male, interactions between these two components of the reproductive system are well known. The study of gynandromorphs of Drosophila simulans led Dobzhansky (1931) to suggest that the spiral form which the testes normally assume in the adult of these animals is only produced when the gonad makes contact with the genital duct. Stern (1941 a and b) investigated the matter by transplanting the testes and amply confirmed Dobzhansky's suggestion, finding that the genital duct induces the testes to grow into a form which, surprisingly enough, is characteristic of the inducer and not of the reacting material. Further, Stern & Hadorn (1939) have revealed a reciprocal reaction by which the testis affects a duct to which it is attached in so far as it supplies to the latter pigmented cells which clothe it in a coloured epithelium.

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