Abstract Fragments from the imaginal wing disc of Drosophila melanogaster were cultured in vivo for periods up to 28 days. One type of edge fragment first duplicated and then ceased to grow, but others often continued to grow following initial duplication and regenerated structures characteristic of other areas of the disc. After 28 days of culture, about 50% of fragments from the presumptive ventral hinge region of the disc grew extensively and produced regenerated as well as original structures. The regenerated structures in some implants were produced at the line of mirror-image symmetry. Regeneration was associated with fragment growth and in many cases was accompanied by loss of duplicate structures. Fragments which were only duplicated after the culture period could in some cases be stimulated to grow by additional culture in fresh hosts, but the results of coculturing two fragments in each host show that culture conditions alone do not control growth and regulation in the fragments. The large, normally regenerating fragment, complementary to the ventral fragment, did not appear to grow following regeneration and only occasionally produced supernumerary structures during prolonged culture. Intact wing discs cultured under similar conditions never produced supernumerary structures. Our results suggest that a duplicated pattern is less stable than a complete, regenerated pattern, which in turn is less stable than an intact disc. We propose that the growth of duplicated disc fragments is stimulated by polarity reversals present at lines of mirror-image symmetry.