In situ RNA hybridization on horizontal sections in wild-type, Fgf8Null/Neoand Fgf15-/-E9.5 embryos. (a, b) Fgf15 (a) and Fgf8 (b) expression in adjacent sections of a wild-type embryo. (c-d') Fgf15 expression in wild-type (c) and Fgf8Null/Neoembryos (d). Higher magnifications of (c, d) are shown in (c', d'). (e-n) Expression analysis in wild-type and Fgf15 mutant embryos showing adjacent sections of CoupTF1 (e, f), Fgf8 (g, h), Spry2 (i, j), Erm (k, l) and cMyc (m, n). Arrowheads in (a, b): rostral midline that is Fgf8+ and Fgf15-; the midline becomes Fgf15+ in Fgf8Null/Neo(d, d'). Arrow in (d): loss of Fgf15 expression in the midbrain of the Fgf8Null/Neomutant. Arrows in (e, f): reduced CoupTF1in Fgf15-/-. Bar in (a) is 200 μm.