Contributors to this symposium focus on the interface between genes and cells, covering genetic analysis, cloning studies, and the investigation of cell lineages and cellular interactions. They note how the body axes are already determined in the eggs of invertebrates and amphibia, then consider the mechanisms as the egg cleaves, in annelids, arthropods, amphibia, and mice that underlie assignation of cells to specific lineages, which give rise to different tissues in the adult. Closing chapters characterize the molecules that mediate each cell"s particular fate, its position in the final body plan as the result of cell sorting or, in some cases, cell migration.