Fig. 1. BDM exposure late in the first cell cycle produces counterclockwise torsion of blastomeres during first cleavage. (A) Xenopus embryos were immersed in 20 mmol/l BDM at 75 minutes post-fertilization (0.83 NT). In every embryo, each blastomere rotates relative to the other in a counterclockwise direction, resulting in a characteristic chiral pattern. (B) The counterclockwise twisting of the pigmented animal cortex of each blastomere relative to the animal-vegetal axis of the embryo (black arrows) produces an invariably counterclockwise torsion between blastomeres as they cleave (blue arrows).