In the original two cilia hypothesis, the authors suggested the presence of two distinct populations of nodal epithelium:nodal pit cells with motile cilia(possessing both left–right dynein and polycystin-2[PC2])located centrally, and the perinodal crown cells with sensory cilia(possessing PC2 but not left–rightdynein)located circumferentially. In our modified model,non-motile 9+2 cilia are speculated to be installed in the circumferential area of thenode,