Spatial and Temporal Changes in Early Amphibian Development

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Abstract
Following fertilization in Xenopus eggs, the dorsal-ventral asymmetry of the egg is established and a rapid cell cycle is begun. In the mid to late blastula period this initial asymmetry is translated into differentiation of the dorsal mesoderm initiated by the vegetal dorsalizing center; as suggested by experiments of Nieuwkoop. About this time the cell cycle undergoes a modification coincident with several events including the onset of cell motility. We report here on experiments describing how the dorsal-ventral asymmetry is produced in the early period. In particular we discuss the role of the sperm, its associated aster, the cortex, and redistribution of cytoplasmic contents. This analysis furthers our understanding of UV effects on dorsalization and the mechanism of twinning. We report also on evidence for a cytoplasmic clock regulating the cycle of DNA synthesis and cytokinesis in early cleavage, as well as on further experiments on the mechanism of cell cycle changes in the midblastula period.

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