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Autonomous development of parts isolated from primitive-streak-stage mouse embryos. Is development clonal?

The relationship between growth rate and regionalization of amphibian, bird and mammalian embryos is briefly reviewed. In contrast to the others, mammals start gastrulation with few cells but accelerate cell proliferation coincidentally. Experiments are described which demonstrate (1) autonomous development of pieces isolated surgically from such mouse embryos, and (2) an absence of regeneration or regulation. Since such embryos regulate completely after chemically induced random cell death it is postulated that these results reflect developmental determination and a resulting mosaicism that suggests development may have a clonal basis. Maps are drawn, allocating positions to various tissues in the embryo.