["Anlageplan", determination and capacity for transdetermination of the male foreleg disc of Drosophila melanogaster].

1. Various fragments of the disc of the foreleg of the late third instar were implanted individually into host larvae of the same age. The imaginal discs ofDrosophila melanogaster have a mosaic determination. Therefore it is always possible to correlate certain adult structures with particular fragments. Primordia of morphologically well distinguished elements such as the claws, sex comb, transversal rows and different groups of sensilla were localized in an anlageplan. 2. It has been shown from the arrangement of the primordia that the disc evaginates in a telescopic fashion during pupal development. 3. In the trochanter the anlage of a bristle organ (edge bristle) was localised. The arrangement of the sensilla and the partial hairiness allow a definite identification of this structure even in small fragments. 4. If we divide the primordia of the edge bristle from discs of thelate third instar and implant these fragments into 72-80 h old larvae the preparations rarely show differentiation of the edge bristle. It seems that at this stage only a small number of cells is competent to form this structure. The edge bristle appears more often if the cells of these fragments are able to proliferate before metamorphosis. 5. The complementary halves of asingle disc can be followed. Young fragments differentiate an edge bristle in each partner much more frequently than fragments of old discs (115 h). Therefore, the cells must lose the capability to form an edge bristle between 72 and 115 h. 6. Transdetermination of leg discs after culture in adults for 14 d lead to the following adult structures: spread of wings, basis of wings, thorax, antenna and proboscis. Elements derived from the wing disc appear with the highest frequencies. 7. Transdeterminations do not equally arise from all of the cells of the leg primordia. Elements of the arista e.g., originate only from the distal parts of the leg anlage. 8. Old donor discs deliver allotypic structures more frequently than young ones. Even fragments from 5-6 h old prepupal discs can still transdetermine. 9. In the discussion it is proposed that transdetermination occurs under physiological conditions found at a late larval or early pupal stage.