The Comparative Embryology of the Diptera

Abstract

In the literature of a few years ago, the dipteran egg was always de­ scribed as a mosaic egg of the classical type (184). It was also thought that the investigation of dipteran embryology, while providing insight into the epigenesis of the larval organization, bore little relevance to the epigenesis of the adult. Recent work has disproved both of these assertions. Striking regulative phenomena have been revealed in the eggs of Nematocera (41, ISS, 190, 191) and a fundamental development of the adult organization has been displayed during embryogenesis (6, 7). The manner in which oogenesis creates the prepattern from which epigenesis proceeds has also yielded partially to analysis in recent years (8, 77, 78, 117, 118). In consequence, our understanding of comparative dipteran developmental morphology, of its epigenetic causation and of its oogenetic background has now attained a stage which raises many new questions. Their solution calls for the use of more refined techniques than have been available for dipteran eggs hitherto, but the introduction of electron microscopy (132-134, 144, 164, 179, 185), cytochemistry (41, 117, 192, 193), and especially tissue culture (129), into this field of investigation holds promise of exciting advances in the next few years. In orientating the present review toward a summation of the current understanding of comparative dipteran embryology, it has been necessary to curtail reference to certain recent advances only to aspects pertinent to general epigenesis. This approach has been adopted toward the develop­ mental effects of mutations disturbing embryonic development in Drosophila (25, 32, 33, 34, 37, 39, 44, 45, 48-53, 61, 62, 63, 84, 85, 108, 114, 147-149, 175, 178, 183, 189), to chromosome elimination during cleavage in cecido­ myid and sciarid Nematocera (11,13,47,76,90,91,137,139), and to investi­ gations of haemocoelous, parthenogenetic, paedogenetic development in cecidomyids (86, 90-93, 110, 113, 126, 127, 159). Each of these, while it has its own appeal, is peripheral to the main theme under discussion.