Genetic mosaics produced by selective chromosome loss or X-ray induced mitotic recombination have been studied extensively in Drosophila (for review, see Gehring, 1978). A consistent finding is that clonally related cells tend to remain in contiguity throughout development in this arthropod. Use of exogenous molecules such as horseradish peroxidase (HRP) to label single cells in the early embryo of the leech has revealed that clones also exhibit coherent growth in this invertebrate (Weisblat, Kim & Stent, 1984). Using essentially the same approach, Kominami (1983) found that little intermingling of cells occurred in the starfish embryo up to the early bipinnaria larval stage, except in the migratory mesenchyme that originates during gastrulation. Exhaustive lineage studies have shown that spatial relationships between cells are also largely conserved during development of the nematode worm, Caenorhabditis elegans (Sulston, Schierenberg, White & Thompson, 1983). Investigations using various extrinsic cell markers lead to...

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