Fig. 7. Mean square displacement analysis of the nine particle trajectories (A-I) shown in Fig. 4. The essentially linear nature of the initial trajectories indicates random motion. When a particle approaches the edge of the head (as in B) or the boundary with the postacrosome (as in H), it becomes non-random due to restricted movement in the x or y axis. This coincides with a succession of short jumps before the particle moves away from the boundary.