Abstract

Some aspects of insect development were reconsidered in relation toWolpert's concept of “positional information”, which was briefly summarized. His distinction between positional information and “polarity potential” was shown to be unnecessary. The question was discussed whether transdetermination inDrosophila imaginai discs is a re-specification or a re-interpretation of positional information. In the first case transdetermination would depend on spatial relationships in the blastema, whereas in the second case it would not. As to the so-called “prepattern mutants”, it was emphasized that in all cases published so far the supposed prepattern alterations are accompanied by growth disturbances which may account for changes in the specification of positional information. “Regulation” in imaginai discs is of a particular kind in that it depends on a proliferation phase. It takes place according to rules which were newly formulated in terms of positional information; the salient point of these rules is a tendency to maintain the slope of the positional information “gradients” and to produce symmetrically reduplicated, often defective organs. A short description of the postulated determinative processes occurring in imaginai discs from the early embryo up to the pupal stage was given. Intermingling of cells from various discs has shown that the cells may restore the original patterns in so far as their positional information has been stably specified,i.e. up to the level of the organ districts; within the districts mosaics are formed, which shows that positional information is specified with respect to new reference points. Transplantation experiments involving the abdominal epidermis of some other insects point to the possibility of limited local shifts in the positional information, and of both increase and reduction of the slope of the positional information curve