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. Actin is the protein driving these cell shape changes, the same protein involved in muscle tightening, and a constituent of the cytoskeleton. The role of Rho1 and DRhoGEF2 are dealt with in the Biological Overview of Rho1 . Both snail and twist , regulated by Dorsal , are required for successful completion of gastrulation.