The first cleavage plane and the embryonic axis are determined by separate mechanisms in Xenopus laevis. II. Experimental dissociation by lateral compression of the egg.

In eggs of Xenopus laevis, the meridian of sperm entry (SEP meridian), the direction of subcortical rotation, and the first cleavage furrow have been used to predict, with varying degrees of accuracy, the position of the plane of bilateral symmetry of the embryo. We show here that altering the shape of the uncleaved egg by lateral compression disrupts some of these topographical relationships in a reproducible way. The neural groove, which identifies the embryonic dorsal midline, usually forms at either of the two narrow ends of the compressed egg, regardless of the position of the SEP meridian, whereas the first cleavage furrow divides the compressed egg across its shorter dimension, regardless of the position of the SEP meridian. Thus the positions of the SEP meridian, the cleavage plane, and the embryonic bilateral plane can be completely uncoupled from each other. In contrast, the direction of subcortical rotation is usually parallel to the plane of compression and predicts the position of the neural groove in all cases. Since the direction of subcortical rotation and the plane of bilateral symmetry still correlate under conditions of compression, we conclude that subcortical rotation is the crucial early step in the process of axis specification.