The unfolding of any new life is a wonder to behold, and even more compelling when understood at the biochemical or genetic level. In the fly, it is overwhelming. There is not just a single site for cell invagination, but taken together, one finds approximately ten morphogenetic movements, three of which can be considered gastrulation proper, and seven more that should be understood in order to understand Drosophila development as a whole. The initial structuring or ground work for most of these events can be traced back to the four maternal systems which establish polarity in the egg and, as a consequence, in the zygote. Thus these events are related to patterns of segmentation built early in development.
Gastrulation begins three hours after fertilization.
By this time there have been 13 mitotic cycles. Immediately after cellularization, a process taking less than an hour from start to finish, the ventral furrow, which marks the beginning of gastrulation, begins to form. Invagination is driven by cell shape changes. Cells in the midventral region undergo an apical constriction. As a consequence of this change in cell shape, adjacent cells are pulled toward the inturning, developing furrow.