Myosin-2 is important for constriction in phase 1, but not in phase 2. (A–F) Live sqh^WT (black) and sqh^AA (green) embryos. (G–L) Live wild-type (WT; black) and rok² (pink) embryos. (A and G) Rings visualized with Sqh-GFP (A) or G-actin^Red (G) constricting over time (minutes). Bars, 10 µm. (B and H) Ring perimeter versus time. (C and I) Constriction rate per phase. (D and J) Circularity 40 min after cellularization onset. (E and K) Switching time from phase 1 to phase 2. (F and L) t_1/2 for F-actin in photobleached rings (G-actin^Red; F, n ≥ 10 rings from four or more embryos per genotype; L, n ≥ 26 rings from ≥12 embryos per genotype; mean ± SE; see Fig. S1 and Table S1). (A, B, G, and H) Gray shading highlights phase 1 in wild-type embryos. (G) Wild-type images are the same as in Fig. 4 A and Fig. 5 (A and G). (B–E and H–K) n = 6 embryos per genotype, 5 rings per embryo; mean ± SE. (C–F and I–L) *, P < 0.05; n.s., not significant.