Caenorhabditis elegans is a small nematode worm. In the wild it lives in the soil and eats bacteria. Sydney Brenner chose C. elegans in the early 1960s as a “model organism” for biological research, particularly the genetic control of development and the nervous system. He was interested in a simple animal that was easy to grow, easy to observe, and easy to work with genetically. Caenorhabditis elegans grows quickly in the laboratory (it takes 3 days to proceed from a newly fertilized egg to a reproducing adult). This worm is transparent at all stages of development, so it has been possible to observe C. elegans from egg to adult in exquisite detail. One fascinating outcome of this study has been the realization that development in C. elegans is strongly determined; that is, each adult has very nearly the same number of cells, and in development these cells arise at the same time and in identical patterns. The complete cell lineage of C. elegans, from single-celled fertilized egg to 959-celled adult female, is known. Male adults have an additional set of muscle and nerve cells used in mating, making 1031 cells in all.