The aim of this program is to analyze the basic mechanisms of development in multicellular animals. We are concerned with developmental processes of a supracellular nature such as intercellular communication, pattern formation, and morphogenesis as well as with essentially intracellular processes such as determination. These processes occur in all multicellular animals including man, but the mechanisms are still entirely unknown. We believe that genetic methods have unparalleled power for analyzing these processes and have focused our efforts on the fruit fly Drosophila which is ideal for genetic manipulations and permits experiments which are not possible with many of the usual animals used by developmental biologists. We are studying the organization of the egg of Drosophila in relation to its subsequent embryonic development, by observing the consequences of various genetic defects in egg development. We are also studying the problem of pattern formation; that is, the origin of spatial patterns of differentiation during development. For these studies we use the imaginal discs of Drosophila, employing a combination of surgical and genetic techniques. We are also extending certain of the ideas developed in Drosophila by the use of hemimetabolic insects, and we are testing the generality of the main concepts by a series of experiments on amphibian limbs. The results obtained thus far encourage the belief that these systems are uniquely suited to answering some fundamental problems of development which are difficult or intractable in mammalian systems.