Fig. 9. A model for notochord formation. BMP signals are blocked in dorsal mesoderm in both somitic (yellow) and notochord (orange) territories. Wnt signals are also blocked in the notochord region, allowing maintenance of Xnot expression. This state is maintained by the antagonistic action of Frzb, secreted from the prechordal plate (red). frzb expression is positively regulated by Gsc.