Fig. 8. Schematic drawing to show the organizing activity of Fgf8 and its signal transduction. Fgf8 is induced at the interface of Otx2 and Gbx2 expression, overlapping with Gbx2 expression. The site where Fgf8 mRNA is localized may receive a strong Fgf8 signal and cause the Ras-ERK pathway to be activated. Thus, this region may acquire the characteristics of rhombomere1 (r1), where cerebellum differentiates. By contrast, in the mesencephalon, the Fgf8-Ras-ERK pathway may be activated only weakly, which may play a role in rostrocaudal polarity formation of the tectum.