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Fig. 6. Shh signaling is confined to ventral brain regions. Shh (A,C) and Ptch (B,D) expression were examined at the 14- (A, B), 24- (C) and 21-somite (D) stages. Ptch was not detectable in any dorsal parts of the brain. Note that lateral expansion of Shh expression in the ZLI initiates at the 24-somite stage (C, arrow). Arrowhead indicates the diencephalon-midbrain boundary.