Cortical protrusion and spindle midzone-induced membrane furrowing during polar body extrusion. (A-D) Different time points after SrCl2 treatment showing the cortical cap protrusion (arrowheads) and the spindle midzone-induced membrane furrowing (arrows). Note the cortical protrusion overlying the chromosomes (arrowheads) and the spindle midzone-induced membrane furrows changing from the initial unilateral (B and C, arrowheads) to the eventual bilateral (D, arrowheads). (E) Cortical cap protrusion (arrowhead) after disruption of spindle microtubules by nocodazole. (F) Block of cortical protrusion by BAPTA/AM. (G, H) Block of cortical protrusion by blebbistatin and ML-7 during egg activation.