Formation of bilateral symmetry. Three-dimensional models of embryonic cell nuclei. In C. elegans, symmetry formation within individual lineages is not very apparent in early stages due to oblique division of somatic founder cells, but becomes successively more obvious even though not perfect. This is exemplified by the behavior of the S3 cells (green spheres; (a-l)). In contrast, P. sambesii performs strict left-right divisions of founder cells, generating in this way an essentially perfect bilaterally symmetric early embryo (m-x). Color code: S1 (AB), blue; S2a (MS), orange; S2p (E), yellow; S3 (C), green; S4 (D), purple, germline (P), red.