Cytodifferentiation may be viewed as a succession of developmental phases (predifferentiation, protodifferentiation, peak differentiation) whose temporal boundaries are defined by discrete transition points. The proposed program is designed to exploit insect exocrine glands as model systems in which to explore and define the mechanics of transitions in cytodifferentiation and with which to analyze those mechanisms that trigger the transitions and that modulate developmental processes during the intervening phases. Differentiation of the accessory spermathecal gland (ASG) of female mealworm beetles occurs over a period of twelve to fourteen days. Within this time span, a stubby sac of superficially unspecialized cells undergoes one or more mitotic divisions, changes in shape to form an elongate tube, lays down an elaborate system of cuticular ducts, and (during the phase of peak differentiation) elaborates large amounts of cell-specific glycoprotein. On both morphological and biochemical criteria, the cells become terminally differentiated. The insect (mealworm) is convenient to obtain and maintain, and has been studied considerably in connection with the endocrine control of its development. Its female reproductive tract is easily manipulated in vivo, and the tract has been maintained in organ culture. We intend to describe the process of cytodifferentiation in the ASG, to define the transition points, and to analyze the control of same by employing surgical, histological, histochemical, ultrastructural, immunological, autoradiographic, and antibiotic-administration techniques.