A simplified model depicting the cortical cap protrusion and spindle midzone-induced membrane furrowing during polar body extrusion. (A) Chromosomes induce formation of a cortical actomyosin cap/ring prior to polar body extrusion. The squared region of the cortical cap/ring is shown on the top, an actin cap (red) surrounded by a myosin II ring (green). (B) Egg activation induces the cortical cap protrusion. (C) The anaphase spindle midzone induces unilateral furrowing. (D) Spindle rotation. (E) Spindle midzone induces bilateral furrowing and abscission of polar body.