Fig. 12. Symmetry breaking accompanies DV axis specification during the first cell cycle. Top: side view. Bottom: vegetal view. (A) Before fertilization, deep (blue) and shallow (red) determinants are localized concentrically about the animal-vegetal axis. (B) After fertilization, the sperm centrosome initiates orientation of the vegetal-cortical microtubule array, producing a shift of dorsalizing components toward the incipient dorsal marginal zone, and a concomitant shift of deeper components and the yolk mass (yellow) toward the sperm entry point. The axial separation of the deep and shallow components defines the plane of bilateral symmetry. (C) An actin-dependent shift of surface counterclockwise relative to the deeper components produces significant material differences across the embryonic midline.