Most theories of determination or differentiation assume that embryonic cells differ from mature cells. Embryonic cells are thought to have metastable control mechanisms. These labile controls are believed to become progressively more stabilized as the cells differentiate. Zygote, blastula, neural plate, limb bud, somite, or ‘stem’ cells are conceived of as undifferentiated, totipotent, or multipotential cells. As such, these cells supposedly have available for activation a larger repertoire of phenotypic programmes than their progeny. A necessary corollary to this view is that the activation of one particular phenotypic programme out of the many available is a function of instructive exogenous inducing molecules.

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