Close Correlation between the First Cleavage Plane and the Body Axis in Early Xenopus Embryos

To examine whether the first cleavage plane demarcates the dorsal-ventral axis (Klein, 1987) or it is random in relation to the plane of bilateral symmetry (Danilchik and Black, 1988), one blastomere of two-cell-stage Xenopus laevis embryos was labeled with injected fluorescein dextran amine. In resulting embryos, the first cleavage plane was represented by the boundary between the labeled and non-labeled cells. In more than 90% of the embryos whose first cleavage plane had coincided with or inclined by 45 degrees to the plane of bilateral symmetry of surface pigmentation, the boundary fell within 30 degrees of the midline at the early gastrula stage. By contrast, in most embryos (10 out of 11) whose first cleavage plane was perpendicular to the plane of bilateral symmetry (up to 0.4% of a batch), the boundary fell between the dorsal and ventral regions. When randomly taken out from batches, more than 90% of the embryos had the first cleavage plane lying within 30 degrees of the dorsal midline at the initial gastrula stage. Thus the present results support neither Klein's nor Danilchik and Black's results, but confirm the currently accepted view that the first cleavage plane marks the dorsal midline with some range of deviation.