Fragments of the wing disc of Drosophila (fig. 2) were either injected into mature third instar larvae for immediate metamorphosis, or cultured in adult abdomens for seven days before being transferred to larvae for metamorphosis. The structures differentiated during metamorphosis were then analysed. The results of the first series of experiments were used to construct an accurate fate map of the disc, and those of the second series were used to determine the regenerative properties of the disc. The fate map (fig. 7) shows presumptive proximal parts (notum, pleura, and dorsal and ventral hinge) at the two ends of the disc, with presumptive distal wing parts in between. During metamorphosis the disc epithelium folds upon itself along the presumptive wing border, bringing dorsal and ventral wing and hinge surfaces into apposition. The wing surfaces occupy a much smaller relative area, and the hinge parts a much larger relative area, in the fate map than in the adult structure. The cultured fragments, in general, behaved in accordance with the rule that when two cut surfaces are created by cutting across the disc, regeneration occurs from one of the cut surfaces and duplication occurs from the other (fig. 14). It was possible to define a level in the longitudinal axis of the disc from which regeneration proceeds outwards. Cut surfaces facing away from this level show regeneration, while cut surfaces facing this level undergo duplication. Similar behavior was found for the transverse axis, and for two diagonal series of cuts. Some fragments with two cut edges could regenerate from one edge while duplicating from the other, whereas others could regenerate from two cut edges simultaneously. However, fragments with four cut edges showed incomplete regeneration, and a high tendency to duplicate even though regeneration in all directions might have been expected on the basis of the other experiments.