Regional variation of poleward microtubule flux in control Xenopus egg extract spindles. (A) X-rhodamine tubulin speckles. (B and C) Speckle trajectories separated by direction of poleward movement. Green and red arrows indicate flux towards the left and right poles, respectively. (D and E) Speckle trajectories color coded according to the velocity histogram (E); the flux rate of the spindle shown is mean ± SD = 2.52 ± 0.65 μm/min (N = 9,562 trajectories). (F) Definition of 23 regions along the pole-to-pole axis. (G) Mean and SD (error bars) of speckle velocities within regions in F. (H) Spatial distributions of normalized speckle velocities of 11 control spindles. Bar, 10 μm.