Domains of gene expression in the gastrula stage chicken embryo. A: Transverse section through the mid-streak region of a stage 4 embryo depicting domains represented by the expression patterns in B-O, B'-O' (green, lateral epiblast; pink, preingression epiblast; lavender, primitive streak; orange, medial mesoderm; purple, lateral mesoderm). B-O: Whole mount ISH localization of mRNAs coding for signalling molecules, receptors and growth factors in stage 4 embryos. B'-O': Transverse sections at the indicated levels through corresponding embryos in B-O. Abbreviations: EPI, epiblast; MES, mesoderm.