[
1990]
The free-living nematode Caenorhabditis elegans is a small and unpretentious organism. It may thrive unnoticed in the cabbage patch in your backyard or the flower pot on your balcony. In their natural habitat soil nematodes live in a thin film of water. In the laboratory C. elegans dwells on Petri dishes in the liquid film on the top of an agar layer, but can also be grown in liquid culture. As in other nematodes the liquid-filled body cavity (pseudocoelom) functions as a hydroskeleton. When the worm dries out, the hydroskeleton collapses and the animal inevitably dies. In a loose sense C. elegans may therefore be considered as a kind of aquatic animal. Because of this and because C. elegans is particularly well suited to the study of certain aspects of development, the following chapter is included in this book on Experimental Embryology of Aquatic Organisms. The intention of this contribution is to serve as an introduction and as a reference source rather than as a complete summary of present knowledge in the field. As indicated by the title, the review will focus on embryonic cell lineages, pattern formation in the embryo and the analysis of mutants affecting early