Embryonic heads were harvested at E12.5 and beads soaked in noggin or BSA were placed as shown in the whole mount photograph in A. After 24 hours in culture, heads were sectioned in the coronal plane and stained for ALP. We show results representative of three repetitions of this experiment. Note that in wild type (B), ALP stain is confined to the frontal bone rudiment in the supraorbital ridge. In the Wnt1-Cre; Msx1/2cko/cko mutant heterotopic bone is visible as a layer of ALP stain extending apically from the rudiment (E). Noggin beads, but not BSA beads, inhibit ALP staining in this layer. Neither noggin nor BSA beads had any effect on the apical mesenchyme of control mice (B, C, D).