Rings constrict in distinct morphological and kinetic phases. (A) Membrane furrows ingress during cellularization. An actomyosin ring (green) constricts to close the cell bottom. (B and C) Fixed wild-type embryos. (B) Rings in surface views (F-actin; phalloidin) at furrow lengths of 1, 4, 7, 16, 24, and 28 µm (left to right). Bar, 10 µm. (C) Ring perimeter (black) and circularity (green) versus furrow length (n = 56 embryos, five rings per embryo; the same trends were found in three independent experiments). Splines show trend of change for data points. Clouds show one SD from the moving mean. (D–F) Live wild-type embryos. (D) Ring perimeter versus time. Rings visualized with G-actin^Red; see Video 1. (E) Constriction rate per phase. (F) Furrow length versus time. (A–D and F) Gray shading highlights phase 1. (D–F) n = 6 embryos, five rings or furrows per embryo; mean ± SE. *, P < 0.05.