Scanning electron microscopy of Drosophila melanogaster embryogenesis : II. Gastrulation and segmentation

Abstract

The sequence of gastrulation events in Drosophila melanogaster, starting with the cellular blastoderm and culminating in a segmented embryo, have been studied with scanning electron microscopy (SEM). Extensive use is made of dissected embryos to illustrate changes taking place within the embryo during gastrulation. During the first 15 min of gastrulation, the mesodermal portion of the germ band is established by the invagination of approximately 1000 cells through the ventral furrow. The primordia for the proctodeum and hindgut are shown to form during early gastrulation. Detailed examination of the surfaces of invaginating primordia shows similarities to other systems and suggests possible underlying mechanisms. Germ band elongation and the formation of the amnioserosa are described. At the time of segmentation, three pairs of rudimentary cephalic appendages develop posterior to the cephalic furrow. Tracheal pits invaginate on all eight abdominal segments and on the second and third thoracic segments. Modifications of the embryonic fate map are discussed.