Embryogenesis of the nematode Caenorhabditis elegans is determinate and virtually invariant from individual to individual. The fertilized egg develops into an anatomically relatively simple juvenile animal having a constant number of only 550 cells (or nuclei) at hatching. The complete embryonic cell lineage up to the 220-cell stage has been described previously, and some lineages have been followed considerably further. During postembryonic development, the cell number increases to about 950 in mature hermaphrodites (including the 143 of the somatic gonad structures) and to about 1025 in males. These cells arise from about 50 blast cells which resume division after hatching. The lineages of all these cells have been described.