Fig. 4. Orthologs of ANR-marking genes are expressed anterior to the CNS. All panels are left lateral views, all insets are frontal. (A) In incipient-tailbud stage, the stomodeal primordium expresses six3/6a. (B) The anterior pharynx primordium and the tail muscle precursors express pitx at incipient-tailbud stage. (C) Schematic overlay of pitx and six3/6a in which the anterior pitx domain lies just interior to the ectodermal six3/6a expression. (D) In mid-tailbud stage the six3/6a ectodermal expression (blue arrowhead) changes shape (see inset). (E) Ectodermal cells begin to express pitx by mid-tailbud stage in weak (purple arrowhead) and strong (purple double arrowheads) domains. (F) Double in situ reveals that only the ventral rank of ectodermal cells weakly expressing pitx overlaps six3/6a expression, while the dorsal pitx-expressing cells do not. (G) Schematic summarizing overlapping (checkered) and non-overlapping pitx and six3/6a domains. (H) A horizontal row of ectodermal cells expresses eya in mid-tailbud embryos. (I) Schematic shows inferred overlapping (checkered) and non-overlapping eya and pitx domains. ec, ectoderm; en, endoderm; gi, endodermal gill pouch primordium; ph, pharynx primordium. Black arrowhead indicates separation between trunk and tail. Scale bars: 20 µm.