Position of early S1 (AB) descendants and handedness of adults. (a-e) Three-dimensional models of embryonic cell nuclei. In C. elegans, the four descendants of S1 form an early left/right asymmetry, (d) resulting in a specific arrangement of gut and gonad ((f), ‘normal development’). Experimentally induced mirror-image arrangement of S1 descendants (e) results in a ‘situs inversus’ (g). In Plectus sambesii, positions of S1 cells differ considerably (a-c), but all variants develop into adults with normal handedness (f). For nomenclature of spatial variants, see Figure 4.