The developmental potentials of dissociated cells of the different regions of the male foreleg disc ofDrosophila melanogaster were analysed. To this end, various amounts of foreleg disc material were dissociated together with an excess of heavily irradiated wing discs ("feeding layer"), and the reaggregates were cultured for 10 days in the abdomens of adult hosts prior to metamorphosis. 2. The foreleg disc cells were in most cases unable to regenerate missing structures in a circular direction within the leg segments. Instead they strongly tended to adopt the specifications of more distal leg segments (distal transformation), irrespective of the region of origin of the ancestor cells within the disc. 3. The distal transformation occurred mainly, if not exclusively, during an early phase ("initial phase") in the reaggregates. 4. The extent of distal transformation was most pronounced in those series in which the foreleg cells were initially least diluted by the "feeding layer" cells. 5. Cells of the lower lateral quadrant were very poor both in proliferative activity and in the extent of distal transformation, compared to cells of the three remaining quadrants. In the experiments with a low initial dilution of the foreleg cells, cells of the lower medial quadrant underwent distal transformation much more distinctly than cells of the upper medial and the upper lateral quadrants. 6. Allotypic structures occurred exclusively in reaggregates of the upper medial and upper lateral quadrants. In these implants, however, the frequency of transdetermination was extremely high. 7. Two alternative mechanisms are discussed which could have led to the general occurrence of distal transformation. They differ in the basic assumption of whether or not the "feeding layer" cells were able to interact with the leg cells to influence their regulative behaviour. In addition, interactions among the leg cells themselves seemed to stimulate proliferation to varying degrees and may account for the observed differences in the degree of distal transformation.
