In most species the cell cycle is arrested in the unfertilized egg. After fertilization the cell cycle is reestablished and a rapid series of cleavages ensues. Preceding the first cleavage in Xenopus the egg undergoes a contraction of its cortex, called the "surface contraction wave," which can be visualized by time-lapse cinematography. This wave of contraction is propagated in a circular manner from the animal pole to the equator. We have found that eggs prevented from cleaving by treatment with antimitotic drugs undergo a sequence of periodic surface contraction waves timed with the cleavage cycle in untreated eggs. In addition, artificially activated eggs, which fail to cleave presumably for lack of a functioning centriole, undergo the same periodic contractions. No nuclear material is required for the periodic waves because a separated egg fragment, produced by constricting a fertilized egg, still undergoes contraction waves with the same period as the cleaving nucleated fragment. These results demonstrate that some expression of the cell cycle persists in the absence of any nuclear material or centrioles, suggesting to us that a biological clock exists in the cytoplasm or cortex of vertebrate eggs, which may be involved in timing the cell cycle.