Fig. 1. Translocation of the maternal dorsalizing activity. (A-D) Translocation of the dorsalizing activity leads to ß-catenin stabilization and the formation of the dorsal organizer. Between the time of fertilization and the first embryonic cell division, a maternally deposited dorsalizing activity (red) moves from (A) the vegetal pole to (B) the prospective dorsal region. (C) By the two-cell stage, maternal ß-catenin (yellow) has become asymmetrically stabilized in the region that has received the dorsalizing activity. (D) Stabilized ß-catenin activates genes of the dorsal organizer (green circle; also called the Nieuwkoop and Spemann organizers) in the dorsal equatorial region, as shown in an early gastrula embryo. (E,F) The dorsalizing activity translocates in the same direction as cortical rotation. (E) The dorsalizing activity (red) resides in the shear zone, an area of looser cytoplasm that forms between the outer cortex of the egg and the dense core cytoplasm following fertilization. The black bars at the vegetal pole mark the starting positions of the core and the cortex early in the first cell cycle. (F) During the first cell cycle, the cortex rotates relative to the core, moving about 30° towards the dorsal side in the same direction as the dorsalizing activity. This process is called cortical rotation, as represented by the displacement of the outer black bar. D, dorsal; V, ventral.