Fig. 1.

(A) A secondary axis can be induced in developing Xenopus embryos by injection of RNA encoding β-catenin into a ventral cell of 4-cell stage embryos. Ventral cells are usually distinguished by their larger size and darker pigment compared to dorsal cells. For detailed methods see (Kuhl and Pandur, 2008a). (B) The duplicated axis is visible in neurula stage embryos within 2 days of injection. Embryos in these images have undergone in situ hybridisation for neural-β-tubulin to illustrate the bilateral stripes of primary neurons and trigeminal ganglia. Embryos can be exposed to a range of compounds during development to assay for ability of the compound to inhibit axis duplication. Alternatively, RNA encoding proteins of interest can be injected into the ventral cells to assay for ability of the protein to induce a secondary axis.