Embryonic heads were stained in whole mount (A-F) or in cross sections (G-L) for alkaline phosphatase (ALP activity at E12.5 (A, B, G, H) and E13.5 (C, D, I-L) and E14.5 (E, F). At least three mutant and three control embryos were examined at each stage. Shown are representative images. Dotted lines in whole mount figures indicate planes of section in Figs G-L. Note that in the Wnt1-Cre; Msx1/2 cko/cko mutant at E12.5, the frontal bone rudiment is not detectible (arrows, B, H). At E13.5, an area of ALP stain is evident apical to the eye, extending approximately 2/3 of the distance to the dorsum of the head (D, arrows). Sections show an ALP positive layer in the apical region of the head (K, L, arrows). Fb, frontal bone; pb, parietal bone; ip, interparietal bone, cs, coronal suture.