Fig. 10. Model for the specification of the lateral rectus eye versus mandibular arch muscles. (A) Schematic representation of a chick head at the onset of EOM and MAM-specific marker gene expression; the non-somitic head mesoderm is depicted in grey. (B) Factors from the neural tube (green) and Fgf8 from the branchial arches (blue) signal to the neighbouring head mesoderm. The signal from the neural tube is accompanied by a further, unidentified signal. As a result of the signalling events (C), the lateral rectus EOM anlage (yellow) is specified next to the neural tube; MAM markers are repressed in this area. MAM (magenta) are specified next to the mandibular Fgf8 signals that suppress EOM markers. lr EOM, lateral rectus extraocular muscle, MAM, mandibular arch muscles.