~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...