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Determination of the dorsoventral (D/V) axis in the frog, Xenopus laevis, begins soon after fertilization, and D/V asymmetry is present in all germ layers from early cleavage onward. Determinants of dorsal axial position may include members of the Wnt gene family and a novel gene, noggin, acting in conjunction with an activin-like protein. Formation of mesodermal tissues is a multistep process. Activins appear to be required during early cleavage stages for initial induction of both dorsal and ventral mesoderm, whereas fibroblast growth factors (FGFs) appear to be required later in embryogenesis and induce posterior (dorsal and ventral) mesoderm. Subsequent to initial induction, the fates of mesodermal tissues are modified by both active dorsalizing and ventralizing forces that operate during late blastula and gastrula stages. Dorsalizing factors may include Wnt proteins, Noggin, activins, and members of the FGF family, whereas ventralizing factors may include the Xwnt-8 protein as well as bone morphogenetic protein-4 (BMP-4 or DVR-4).
Hazel Sive (Fri,) studied this question.