The systematic breaking of left–right body symmetry is a familiar feature of human
physiology. In humans and many animals, this process originates with asymmetric fluid flow
driven by rotating cilia, occurring in a short-lived embryonic organizing structure termed the
node. The very low–Reynolds number fluid mechanics of this system is reviewed; important
features include how cilia rotation combines with tilt to produce asymmetric flow, boundary
effects, time dependence, and the interpretation of particle tracking experiments. The effect …