Expression of sonic hedgehog and FGF8

Lateral view of two stage 15 chick heads showing the regions where the sonic hedgehog gene (right) and the FGF8 gene (left) are switched on. The sonic hedgehog mRNA is being made in the notochord, and the floorplate of the brain and spinal cord, whereas the FGF8 mRNA is found in the most rostral forebrain, the isthmus between the midbrain and the hindbrain and the branchial arch epithelium.