The relationship between cleavage and blastocoel formation in Xenopus laevis. II. Electron microscopic observations.

Blastocoel formation in Xenopus laevis was investigated by light microscopy using serial sections of epoxy-embedded, staged embryos. The earliest manifestation of the blastocoel in the embryo appeared during the first cleavage as a modification in the animal pole furrow tip. This modification consisted of an expansion of a localized area of the furrow. As the blastocoel became a distinct entity, it remained stationary, while the furrow tip continued to advance inwardly. In contrast, no such furrow cavity was observed in the vegetal pole furrow during its formation. During subsequent cleavages, up to the late morula stage, furrows on opposite sides of any given blastomere had different morphologies. As further divisions occurred, the mode of furrow formation became identical regardless of location in the embryo. It is suggested that the cytokinetic pattern in early amphibian embryos is modified to allow for the formation of the blastocoel. After the blastocoel has formed, the cytokinetic pattern changes to one which is concerned solely with cell division.