" . . . but our knowledge is so weak that no philosopher will ever be able to completely explore the nature of even a fly . . . " * Thomas Aquinas "In Symbolum Apostolorum" 079 RSV p/96 This is a monograph on embryogenesis of the fruit fly Drosophila melanogaster, conceived as a reference book on the morphology of embryonic development. A monograph of this extent and content is not yet available in the literature on Drosophila embryology, and we believe that there is a real need for it. Thanks to the pro­ gress achieved during the last ten years in the fields of develop­ mental and molecular genetics, work on Drosophila development has expanQ. ed considerably, creating an even greater need for the information that we present here. Our own interest in embryonic development of the wild type arose several years ago, when we began to study the development of mutants. In the course of these studies we repeatedly had occasion to discover gaps and inade­ quacies in the existing literature on the embryology of the wild type, so that we undertook to investigate many of these problems ourselves. Convinced that many of our colleagues will have encountered similar difficulties, we decided to publish the present monograph. Although not explicitly recorded, Thomas Aquinas was proba­ bly referring to the domestic fly and not to the fruit fly. Irrespec­ tive of which fly he meant, however, we know that Thomas was right.