Acquisition of differentiative competence in the imaginai leg discs ofDrosophila.

When first leg imaginai discs from early third instar larvae are forced to differentiate immediately by transplantation into larval hosts ready to pupate, they fail to differentiate all the adult leg structures. Even though the variability among these incomplete differentiations is wide, we were able to draw the following conclusions. Even if only little differentiations, limited to a few hairs and bristles, were observed, the structures were always identifiable as the most proximal and the most distal structures of the leg. In those cases where differentiation was more complete the tarsal segments were never completely separated; in extreme cases all five segments were fused. The segments more proximal than the tarsus were always separated when differentiation occurred, but not all the structures within the segments were formed. The absence of a particular structure in an incompletely differentiated segment seems to occur in a random way and is not dependent on its location within the disc.