Coplanar cell divisions during gastrulation. (A) Coplanar divisions during Ea/Ep internalization. Time-lapse microscopy with three-dimensional (3D) projection still images. Cytokinetic rings in AB and MS cells are present in frames at 0 and 13.5 minutes, while the white lines at 6 and 13.5 minutes indicate division directions. Note that when AB cells divided and retracted their extensions (frame at 6 minutes), NMY-2 gradually reappeared at the resurfacing part of the endoderm (between frames at 6 and 10.5 minutes). (B) Dynamics of circumferential tissue spreading. (Left) Posterior of a lineaged embryo; circumferential movements are indicated by red dashed arrows. (Right) Quantitative analysis of circumferential tissue movements and comparison to theory; graphs indicate the degree of circumferential spread (mean ± SD). (C) Morphometric analysis of cell-shape changes in the surface layer during Ea/Ep internalization. The graph depicts the change in the ratio between apical and basolateral axis length for three surface cells (ABaraa, ABarap and ABpraa) that were not involved in rosette formation (n = 3, mean ± SD, polynomial fit).