Evidence is given for the opinion that cilia in the early embryo, by their work, determine the laterality of the body; without ciliary work body laterality would be randomized. More exactly, monocilia in the primitive node are responsible for this determination. They have been described as being of the 9+0 type, but with dynein arms and with a gyrating movement. The orientation of the monocilia on the epithelium is of no importance but the direction of their gyration is, as may also be the shape of the node. The chirality of the cilia is thus reflected directly in the asymmetry of the body. The dynein arms go clockwise as seen from the base to tip and the ciliary rotation is in the same direction. The resulting waterflow is towards the left and so is the movement of the forming heart. In most subgroups of the immotile-cilia syndrome this mechanism does not work and equally many individuals will be born with situs inversus as with situs solitus. An exception is the immotile-cilia subgroup, named 'microtubule transposition', which is characterized by all cilia having a 9+0 structure throughout most of their length.