Fig. 5. Time evolution of the three-dimensional cells' distribution by velocities. Surface plot of reconstructed three-dimensional probability density function f (V_r,V_z) (Equation 9), expressed as f (V,), at six time instants. The axes correspond to the cell absolute velocity and the cell velocity angle to the vertical =atan(V_r/V_z). A maximum observed at small V is related to the passive particles suspended in the medium. At about 40 min the number of active swimmers in the observation area becomes large compared to that of passive particles and this maximum vanishes.