Fig. 8. Schematic representation of the repressive effects of the BMP and FGF pathways on endoderm formation. (A) Wild-type context: (a-c) schematic representation of signalling activities of Nodal, FGF and BMP signalling. This representation is speculative and based on the potential range of signals and the expression pattern and range of antagonists (Schier and Talbot, 2005). (B) Formation of the endoderm is negatively regulated by a combination of FGF and BMP signals. (C) DN-Ras overexpressing embryos: inhibition of MAPK signalling promotes endoderm formation on the dorsal side but causes a loss of endoderm precursors on the ventral side owing to increased expression of the BMPs. (D) Triple inhibition of FGF signalling with morpholinos or combined inhibition of Ras and BMP signalling with DN-Ras + noggin: simultaneous removal of the BMP and FGF dependent inhibitions promotes endoderm formation all around the embryo.