(color online) (a) trajectories generated by PRW model (upper panel) and APRW model (lower panel), and the corresponding primary directions . (b) Polar plots of the velocity magnitude at different orientation angles with respect to the primary direction of cell trajectories. The blue circle is for PRW model at and the red ellipse is for APRW model at and . (c) Velocity magnitudes versus orientation angle. (d) Semi-major and semi-minor axes, as well as the area of the ellipse, against the persistent time P_x.