Figure 9.

 Localization of the endogenous dorsalizing activity in the embryo. (A) Diagram of a 32-cell stage Xenopus embryo showing the four tiers of blastomeres. (An) Animal pole; (Veg) vegetal pole. (B) The endogenous dorsalizing activity is highest in dorsal vegetal cells. A single dorsal or ventral blastomere at the 32-cell stage was injected with S01234. The luciferase activity of a pool of five embryos injected into the same blastomere indicates that the endogenous dorsalizing activity is present mainly in the dorsal C and D tiers (dorsal vegetal cells) of early embryos. This region of the embryo is most abundant in β-catenin (Larabell et al. 1997).