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Since Aristotle, humans have pondered how body patterns form. Almost invariably, each person has the same skeletomuscular arrangements. Some bodily structures, such as vertebra and ribs, seem to come about by reiteration of a common process. These segments derive from somites, which are serially repeated precursors that in turn develop by sequentially 'budding off' from the developing embryo's undifferentiated presomitic mesoderm (PSM).

Somitogenesis occurs like clockwork, and in 1976, an oscillatory mechanism was proposed to account for it.1 In 1997, molecular evidence of an oscillator in vertebrate PSM was uncovered, found in a chicken homologue of the Drosophila segmentation gene hairy. This gene--coding for part of the Notch signaling pathway--was cloned and found to be expressed in cycles, with a period corresponding to the time it takes to...

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