We wish to understand the mechanisms by which genes control development. We are studying two sorts of genetic regulatory hierarchies controlling development in Drosophila melanogaster. One of these specifies the segmentation pattern of the body plan along its anterior-posterior axis. Within this hierarchy we are mostly concerned with the "homeotic" genes which act as general transducers of positional information to specify the segmental identities-i.e., the characteristics that distinguish one metamere from another. The second kind of hierarchy serves to coordinate temporally the developmental pathways of different tissues at various stages of development.