ABSTRACT
The egg of D. tryoni undergoes 7 synchronous nuclear cleavages before 90 nuclei invade the surface. The surface nuclei then undergo 6 further synchronous cleavages and a blastoderm of 5,800 cells is formed. The 38 nuclei remaining in the yolk divide synchronously three times to give about 300 primary vitellophages. At the posterior pole 4 pole cells are cut off, dividing synchronously three times to give 32. 2-5 nuclei at the posterior pole form secondary vitellophages.
Gastrulation, involving invagination of anterior and posterior mid-gut rudiments, is accompanied by invagination of mesoderm mid-ventrally and of pole cells and proctodaeum posteriorly. The dorsal blastoderm thins to extraembryonic ectoderm, displaced laterally as the germ-band elongates during gastrulation. Elongation takes place mainly posterior to a temporary cephalic furrow, behind which a second transverse and four further pairs of temporary lateral folds form. Most of the vitellophages migrate to the yolk surface to form a nucleated yolk sac.
Organogeny is accompanied by segmentation, shortening of the germband, dorsal closure, and involution of the head. The gut develops from stomodaeum, proctodaeum, and anterior and posterior mid-gut rudiments. Malpighian tubules arise as outgrowths of the proctodaeum, salivary glands as ventrolateral ectodermal plates on the labial segment. The paired mesodermal bands do not segment. Splanchnic mesoderm gives rise to visceral musculature, somatic to segmental musculature, fat-body, heart, and gonad sheaths. The primordial germ cells of the gonads are formed by 16 pole cells. Other pole cells form part of the proctodaeal wall. The central nervous system arises from ventro-lateral and antero-lateral ectodermal neuroblasts separated from the hypodermis, the tracheal system from paired segmental ectodermal invaginations. The remainder of the embryonic ectoderm gives hypodermis; extra-embryonic ectoderm is resorbed at dorsal closure. No amnion or serosa form.
Cleavage in D. tryoni is typical of Diptera. The rate of synchronous cleavage and the mode of formation of the blastoderm follow a relatively constant pattern in different species and although the number of cleavage divisions, timing of nuclear invasion of the egg surface and number and mode of origin of the pole cells and vitellophages vary, the resulting blastodermal structure and presumptive areas in the blastoderm are probably constant for the order. The mode of gastrulation is also constant, save for a difference in pole-cell behaviour in Nematocera and Cyclorrhapha. In organogeny, the pole cells of Cyclorrhapha generally contribute to the gut-wall, those of Nematocera do not, due to the relative timing of pole-cell formation and determination of the presumptive areas of the blastoderm in the two groups. Gut formation in D. tryoni is otherwise typical of Diptera, as is the further development of the mesoderm. Segmentation in Diptera is ectodermal, tending to be suppressed in the mesoderm. Preoral segmentation is wholly suppressed. The germ cells of dipteran gonads always arise from pole cells, the gonad sheaths from mesoderm. In Nematocera, the extra-embryonic ectoderm extends as amnion and serosa; in Cyclorrhapha no embryonic membranes develop. Cell lineage of the larval organs of Diptera is more or less constant among species.
