ull recognition of the important role that hormones play in the growth, F differentiation, and function of insects was preceded by rather advanced information about identity and action of mammalian hormones. However, extirpation and transplantation of endocrine glands in insects and segmental ligation of larvae gradually yielded knowledge about the location of endo­ crine cells in insects and led to labeling their hormonal product with appropriate names. The development of relatively sensitive assays permitted the separation of active extracts of the hormones associated with meta­ morphosis and quickened the pace of discovery. Finally, identification of structural formulae of natural hormones now has provided the means for detailed study of biochemical events with which they are associated. In the meantime, better interpretation of the mechanism for morphologic and functional effects of hormones in vertebrates has come about through knowledge of binding of hormones in target tissues, feed-back, and other mechanisms. There has been a prodigious increase in information recently, not only about the structure of molting and juvenile hormones, but also about their molecular action, as well as recognition of problems still posed by their of origin, and relation to genic derepression. biosynthesis, interactions, sites Discovery and study of phytohormones and analogs has added interest and amplified techniques for investigation.