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Fig. 2 Expression of pnx. Expression of pnx was detected by whole-mount in situ hybridization. (A,B) 50% epiboly stage. Lateral view (A) and animal pole view (B). Dorsal is towards the right. pnx was expressed in the entire ectoderm exclusive of the marginal blastoderm (arrowhead in A). (C-F) 70%-epiboly stage. Dorsal view (C), lateral view (D), animal pole view (E) and sagittal section of the dorsal marginal region (F). (D-F) Dorsal is towards the right. The pnx transcripts were detected in the ventrolateral and dorsomedial regions of the ectoderm at the mid-gastrula stage. (G,H) 85% epiboly stage. Dorsal view (G) and lateral view (H). (I-K) One-somite stage. (I) Dorsal view. (J) Two-color staining with pnx (purple) and krox20 (red). (K) Horizontal section of the pnx-stained tail-bud state embryos (m, primary motoneurons; i, primary inter neurons; r, Rohon-Beard neurons). From the late gastrula to early segmentation stages, the pnx transcripts became restricted to three stripes of primary neurons posterior to rhombomere 4 (the krox20 expression indicates rhombomeres 3 and 5). (L) Ten-somite stage (14 hpf), lateral view. (M) Prim-6 stage (25 hpf), lateral view. The expression of pnx gradually disappeared from the anterior region from the late segmentation stages and it was only detected in the caudal neural tube at the early pharyngula stage.