Résumé : The caudal part of the mesonephrogenic mesenchyme of the mouse, which never forms tubules in situ, was combined in vitro either with cephalic fragments of the Wolffian duct (supposed to be inductor of the cranial part) or with inductors of metanephritic tubules : uretral bud, dorsal half of spinal cord. In neither of these combinations has any differentiation of the caudal mesenchyme occurred.