Fig. 8. Schematic representation of Hex interactions with the Wnt and Nodal-related pathways. At blastula stage, Hex amplifies Wnt signalling through the repression of the Wnt antagonist Tle4. The Nieuwkoop centre genes Siamois and Xnr3 are markers of this process and this signalling centre is responsible for the induction of both anterior endoderm and axial mesoderm. Following mesendoderm induction, Hex is expressed in the anterior endoderm where it antagonises the propagation of the Nodal signal to prevent mesoderm formation in the head field. Broken lines indicate the induction of defined domains in the mesendoderm.