Fig. 2. Spaw expression. (A-D) Four to six somite stage. (A) Dig-spaw probe, dorsal close-up of tailbud. (B-D) Two-color in situ hybridization: spaw probe and a spadetail or ntl probe. (B) Dig-spaw (purple) + Fl-spt (red), dorsal view of tailbud. (C) Dig-spaw (purple) + Fl-ntl (red), lateral view of tailbud. n, prospective notochord. (D) Dig-spaw (purple) + Fl-ntl (red), dorsal view of tailbud. (E) Ten to 12 somite stage. Dig-spaw, dorsal view, onset of asymmetric left-sided expression (arrowhead). (F,G) Twelve to 13 somites, dorsal views, comparing spaw (F) and one-eyed pinhead (G) expression. Anterior is towards the top. (H,I) Seven to nine somite stage, lateral view, close-up of heads. Anterior is towards the right. (H) Dig-spaw probe. (I) Dig-oep probe. (J-M) Dorsal views, anterior towards the top. (J) Fifteen to 18 somites, DNP-spaw plus a DNP-netrin probe. Arrowhead indicates left-sided spaw domain. Arrow indicates central stripe of netrin in floor plate. (K,L) Twenty to 23 somite stage. (K) Dig-spaw (purple) showing left-sided spaw domain along most of the length of the embryo. White arrow indicates bilateral spaw flanking tailbud. (L) Left-sided spaw domain has expanded laterally and is fading posteriorly. (M) Twenty-two to 24 somite stage; Dig-spaw probe (inset provides a lateral view of the tailbud).

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