~o working hypothesis amounts to much until it has been tested on suitable material. Indeed, the choice of an appropriate experimental system has often been the key to the solution of a problem. The present volume is devoted to insect imaginal disks. These groups of larval cells are the primordia of precisely characterized adult counterparts, without apparent function in larvae. At the onset of metamorphosis, the subtle interplay of hormonal signals brings growth to a halt, and differentiation begins. In the fruitfly, a host of mutations are known to affect the development of disks; these provide ample material for analysis. It was largely ERNST HADORN'S ingenuity that directed the attention of many scientists around the world to this promising experimental system, and to him this volume is dedicated. All the contributors have been associated with him at one time or another, as graduate students, postdoctoral fellows, or colleagues. Each author has attempted to cover comprehensively the topic assigned to him. This has inevitably led to some overlapping, for which the editors should be blamed, not the authors, as this results from the way the topic was subdivided at the outset. We believe this volume will be a welcome sourcebook for the specialist in the field, and a provocative monograph for the uninitiated scientist interested in the exciting area of cell determination.