1. Two imaginai discs of different mutants, sexes and species respectively of different segments (wing and genital disc) fromDrosophila larvae ready to pupate were mixed in order to get a homogeneous aggregate. Before mixing the two discs were partially dissociated either enzymatically or only mechanically. 2. The aggregates were implanted into larval hosts. Their differentiated structures were studied after metamorphosis. 3. Dissociated cells and groups of cells find together and form organized genital or wing structures. 4. Homoplastic-homonome-homosexual aggregates of male genital discs form mosaics in all plates both donors taking part in building up one plate. The same cooperation is observed in the heteroplastic-homonome-homosexual combination. 5. The homoplastic-homonome-heterosexual combination yields mosaics only in the anal plates and in the region of peripheral bristles. 6. In a heteronome combination the wing and genital cells separate completely so that no mosaic can be formed. 7. Mosaic plates are not randomly composed of different bristle types, but carry only those bristles which occur alsoin situ on one common plate. To form a mosaic, obviously, cells of the same field quality unite (isotypic association), whereby field specificity overrides species, sex and pattern specific differences. 8. In rare cases the rule of isotypic association is broken and "faulty" mosaics are formed. 9. The integration effect and the pattern formation are discussed and interpreted as a result of affinities and movements of determined cells.