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This report marks the completion of a project begun over one hundred years ago -namely the determination of the entire cell lineage of a nematode. Nematode embryos were attractive to nineteenth century biologists because of their simplicity and the reproducibility of their development, and considerable progress was made in determining their lineages by the use of fixed specimens (reviewed by Chitwood and Chitwood, 1974). By the technique of Nomarski microscopy, which is nondestructive and yet provides high resolution, cells can now be followed in living larvae (Sulston and Horvitz, 1977; Kimble and Hirsh, 1979; Sternberg and Horvitz, 1981) and eggs (Deppe et al, 1978; this paper). The use of living material lends a previously unattainable continuity and certainty to the observations, and has permitted the origin and fate of every cell in one nematode species to be determined. Thus, not only are the broad relationships between tissues now known unambiguously but also the detailed pattern of cell fates is clearly revealed.
Our current interest in the cell lineage of a particular nematode, Caenorhabditis elegans, has arisen as part of a larger research effort comprising genetic, anatomical, and biochemical approaches to the development of this animal. The lineage is of significance both for what it can tell us immediately about relationships between cells and also as a framework into which future observations can be fitted.
The main purpose of this article is to present the embryonic cell lineage. A brief description of morphogenesis is given, although this is not intended to be an exhaustive account of cell movements and interactions. A few cell ablation experiments are described; these results are limited in scope, but do give some indication of the developmental flexibility (or lack of it) in the system. In addition, a key to differentiated cell types is provided as an Appendix.
Adapted by Yusuf KARABEY for WORMATLAS, 2003