Embryogenesis of the free-living soil nematodeCaenorhabditis elegans produces a juvenile having about 550 cells at hatching. We have determined the lineages of 182 cells by tracing the divisions of individual cells in living embryos. An invariant pattern of cleavage divisions of the egg generates a set of stem cells. These stem cells are the founders of six stem cell lineages. Each lineage has its own clock-i.e., an autonomous rhythm of synchronous cell divisions. The rhythms are maintained in spite of extensive cellular rearrangement. The rate and the orientation of the cell divisions of the cell lineages are essentially invariant among individuals. Thus, the destiny of cells seems to depend primarily on their lineage history. The anterior position of the site of origin of the stem cells in the egg relates to the rate of the cell cycle clock, suggesting intracellular preprogramming of the uncleaved egg. We used a technique that allows normal embryogenesis, from the fertilized egg to hatching, outside the parent under a cover glass. The cell lineages reported here were analyzed from tapes of two eggs recording the complete development until the animals started moving inside the egg shell. In addition, embryogenesis was recorded and analyzed in 18 other individuals from fertilization to the 30-(10 cases), the 54-, 60-, 75-, and 87-(3 cases), and the 100-(2 cases) cell stages, respectively. We also analyzed the E-cell lineage in six more individuals to make certain that all 8 E-cells divide in going to 16. After recording was terminated, it was ascertained that, under the cover glass, all 26 animals hatched and moved normally.

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