Expression of Cdx2 is essential for posterior neuro-mesodermal axial progenitors Cdx2/T Brachyury double mutants are more severely truncated than single mutants Cdx2 and T Brachyury directly co-activate genes of the Wnt and Fgf signaling pathways The genetic network driven by Cdx/Hox and T Brachyury establishes differential axial growth