1. In last instar larvae pieces of integument from different regions of the sixth segment have been transplanted to the fourth segment. The formation by the grafts of a ridge (region 2) and of region-3 cuticle, normally occurring only in the fifth, sixth and seventh abdominal segments, has been studied in the pupa.
2. Grafts containing presumptive regions 1, 2 and 3 developed these regions when transplanted to an equivalent position in the fourth segment.
3. Grafts containing only presumptive region 1 formed also region-2 cuticle and region-3 cuticle when transplanted to the anterior part of the fourth segment.
4. Grafts containing only presumptive region 3 formed also region-2 cuticle and region-1 cuticle when transplanted immediately posterior to the bristle B1 in the fourth segment.
5. The results support the concentration-gradient concept. They reveal that the gradients in the fourth and the sixth segments are of the same nature and that the difference in the cuticular structure of these segments must result from a difference in competence. The concept of determination is discussed in relation to these results.
Deceased, October 1967. See editor's note, p. 59.