Fig. 2. Normal first cleavage in Xenopus is chiral and not mirror-image symmetric. Both fixed (A-C) and live (D-E) embryos, untreated, reveal a slight counterclockwise torsion of the two blastomeres during cleavage furrow advance. The apex of each furrow margin, i.e. the site at which furrowing began, becomes offset relative to the corresponding point in the opposite blastomere (arrow bases). By midcleavage (C), the offset between devitellinated blastomeres may be as much as 150 µm. Although the asymmetry is accentuated by dejellying (D) or removal of the vitelline envelope, even embryos with undisturbed jelly coats (E) display the same chirality. The offset normally becomes obscured as cleavage proceeds and the stress folds relax.