Embryogenesis in Plectus sambesii . (a) 1-cell stage (P0) with single pronucleus; (b) 2-cell stage with larger somatic founder cell S1 and smaller germline cell P1; (c) T-shaped 4-cell stage after transverse division of S1 and longitudinal division of P1; (c’) rarely S1 divides with longitudinal spindle orientation; (d) diamond-shaped 4-cell stage after rearrangement of blastomeres; (e) 8-cell stage, with gut precursor S2p (yellow) and germline cell P3 (red); (f) approximately 16-cell stage, S2p (yellow) has migrated to the center but keeps contact with the primordial germ cell P4 on the surface (red; mostly out of focus); (g) embryo with 4-cell gut primordium (yellow); (h-k) formation and closure of blastopore (blue) on the ventral surface of the developing embryo; (l) twofold stage (early morphogenesis) with future head region (hr) top and tail region (tr) bottom; (m) early cell lineage, divisions of germline (P1 to P4; changing position indicates ‘polarity reversal’ as described for C. elegans; Schierenberg, 1987) generate somatic founder cells (S1 to S4), the eight descendants of S1 are color coded; S2 divides into pharynx/muscle precursor S2a (orange) and gut precursor S2p (yellow). Arrowheads indicate asymmetric divisions. DIC optics. Orientation: anterior, left. Bar, 10 μm.