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Summary
Among the major milestones in developmental biology are those that authoritatively describe how cells differentiate during development. Conklin (1) described the mosaic development of marine invertebrates (sea squirt), in which specific and recognizable cells of the dividing embryo consistently produce specific tissues of the larva and naturally pigmented myoplasm segregates to the muscle lineage of the larva, supporting the idea of localized cytoplasmic determinants that influence cell fate. Later, lineage tracer dyes were used to track the fates of different regions in amphibian embryos (2). More recently, careful observation produced the triumph of a complete cell lineage map of the nematode worm (Caenorhabditis elegans) embryo (3). On pages 980, 979, and 981 of this issue, Briggs et al. (4), Farrell et al. (5), and Wagner et al. (6), respectively, as well as Fincher et al. (7) and Plass et al. (8), mark another innovation: a description of gene expression in all of the cells of fish and frog embryos (4–6) and in the adult regenerating planarian (flatworm) (7, 8). These papers describe the transcriptional information that forms the blueprints of development and regeneration in these organisms.
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