Fig. 6. Epistasis between FGF, Vg1 and Chordin. When a Vg1 pellet is implanted laterally and an FGF-coated bead grafted on the opposite side 6 hours later (A), FGF induces a primitive streak that bypasses the inhibitor (B). FGF signalling is required for induction of an axis by Vg1: misexpression of Vg1 together with SU5402 (C) does not yield an ectopic axis (D). When Vg1 is implanted laterally, followed 6 hours later by FGF8 beads and a pellet of Chordin-secreting cells on the opposite side (E), the combination of FGF8 and Chordin can bypass the inhibition (F). FGF8 and Chordin can also induce an axis in intact embryos (G,H) [in this case Chordin increases the frequency of ectopic axes with respect to FGF8 misexpressed alone (see text for details)].