As the pluripotent cells of early embryos differentiate, each progressively loses the potency to develop into several phenotypes. Ultimately, each cell becomes irreversibly restricted to the expression of a single phenotype. Although in many instances details regarding those restriction events are well known, there is little information concerning the nature of the gene transcription changes involved. A model that accounts for the diminution of developmental potential as resulting from progressive, irreversible repression of previously active genes is presented. A scheme of progressive gene repression, rather than selective gene activation, is most consistent with observations from experimental embryology as well as from more recent biochemical experimentation.