Movie 1.

Dynamics of Myosin II recruitment The dynamic localization pattern of Myosin II during early NC 14, monitored using a GFP “protein-trap” in the Myosin II heavy-chain (zipper) gene locus, as visualized in cross section by Lightsheet microscopy. During the process of cellularization, Myosin II is initially recruited to the basal aspect of the future cell membranes as they extend from the surface. Upon completion of cellularization, Myosin II disappears form the basal position and is recruited to the apical side in the ventral-most cells, where it drives furrow formation. The graded pattern of Myosin II recruitment within the ventral domain shapes the pattern of cell invagination. The movie was obtained at a rate of 1 frame/min.