A 120 cell cleavage stage sea urchin embryo as viewed from the vegetal pole. Shown is the pattern of ß-catenin in the nuclei of micromeres at the center and in the 16 veg 2 cells surrounding the micromeres. This nuclear movement of ß-catenin is essential for mesendoderm specification. Nuclear ß-catenin starts a series of specification events that establish the important embryonic fates by the end of cleavage.