Medial myosin flows and fuses with AP and DV edges during NB ingression. (A) Classes of medial myosin behavior. Myosin clusters can assemble and coalesce at the cell center (>1 µm away from cell edges) or near (<1 µm) an AP or DV edge and either fuse (second and fourth rows) or not fuse (third and fifth rows) with that edge. Arrowheads track the movement of medial myosin clusters during NB ingression. Bars, 3 µm. (B) Planar distribution of medial myosin clusters. NBs show uniform planar distribution of myosin at the apical cortex, whereas medial myosin is more biased to flow parallel to the AP axis in NICs. P, posterior (0°), A, anterior (180°), D, dorsal (90°), V, ventral (270°). (C) Quantification of medial myosin behavior in NBs and NICs as described in A. n = 268 and 512 myosin clusters in NBs and NICs, respectively (19 cells per condition; seven embryos).