Cilia of Hydin-deficient mice fail to produce a flow. Analysis of cilia-generated fluid flow in the third ventricle (a and b; side view) and in trachea (c and d; top view). The positions of polystyrene particles on selected video frames were marked with colored dots; the distance between neighboring dots is 10 (a), 25 (b and c), or 50 ms (d). The time represented by each trace is noted for some particles. Rapid directional movement was observed with wild-type samples (a and c), whereas flow was absent or severely reduced in the mutant (b and d). Bar, 5 μm. (e) Detailed track of a particle from flow analysis in wild-type brain sections; the distance between dots is 5 ms. The arrowheads mark the periodic changes in velocity and direction of the particle. The arrow indicates direction of movement.