Fig. 7. Model for early mesoderm induction. Maternal β-catenin activity appears to be pre localized along the equator. It is strongest and most widespread in the dorsal side, where it induces dorsal genes, such as Siamois. Both β-catenin and VegT induce Xnrs. FGF, Xnr and β-catenin signals cooperate to induce FGF3 and, perhaps, other FGFs along the marginal zone and thus activate MAPK. The combination of Xnr and FGF/MAPK signals then activates expression of mesodermal genes such as eomesodermin and Xbra.