Intrinsic spindle forces move impaling microneedles through the spindle. (A) Images shown were selected from three time-lapse series of skewered spindles. Dashed lines indicate the position of the microneedle, which remained essentially stationary during the experimental time course. (B) Kymographs were used to analyze velocities of spindles as they translocated off microneedles. Because skewered spindles often rotated during time-lapse experiments as the result of changing flows within the extract (e.g., spindle #3), each image in the series was rotated using custom software to maintain a fixed orientation of the spindle’s interpolar axis (arrowheads mark the position of the needle). (C) The distance between the nearest pole and the middle of the needle was plotted versus time. Slopes were calculated using linear regressions from these plots of pole to needle distances of >5 µm and those equal to 5 µm. (D) Two microneedles were used to impale spindles. In each case, the needles were positioned on the same side of the spindle midzone, and the spindle moved off both needles, regardless of the predominant direction of extract flow (indicated by the white arrow). Dashed lines indicate the position of the microneedles used to skewer the spindles. Bars, 25 µm.