Fig. 2. Gene expression in the primitive streak and tail bud. Embryos were hybridised with in situ probes for Fgf8 (A-B), Evx1 (C-D), Wnt3a (E-F), T (G-H), Foxa2 (I-J) or Sox1 (M). (A,C,E,G,I) Whole-mount lateral views of E8.5 embryos; (A',C',E',G',I') corresponding sagittal sections. (B,D,F,H,J) E10.5 sagittal sections. Black and white arrows: end of the notochordal plate. Inset in G': transverse section of the caudal half of the node (at the level marked in G' by a white arrowhead), showing a cell in the ectodermal layer strongly expressing T. Black box in B: E10.5 chordoneural hinge (CNH). (K-L',N-O) Confocal images showing Sox1-GFP expression (green) and TO-PRO-3 counterstain (red). (K) Sagittal section of an E8.5 embryo. (L) Sagittal section of the tail tip of an E10.0 Sox1-GFP embryo. (L') Magnified view of the CNH region marked (white box) in L. (N,O) Transverse sections through the tail tip of an E10.0 embryo at the levels marked by vertical white lines in L (N, left line; O, right line). (N') Magnified view of the region marked (white box) in N, showing Sox1-GFP-positive cells in the notochord region (arrowhead). (P) Summary diagram. Blue, ectoderm expressing Sox1 and Foxa2; brown, rostral prospective notochord expressing T and Foxa2; orange, caudal prospective notochord expressing Fgf8, T and Foxa2; dark purple, caudal node and streak ectoderm, and caudal ectoderm in the tail bud; light purple, streak mesoderm and TBM, both areas express Wnt3a, Evx1, Fgf8, T and Sox1 (E10.5). Black boxes in P, E8.5 border and E10.5 CNH. not, notochord; nt, neural tube; hg, hindgut; np, neural plate; ps, primitive streak; tb, tail bud; tbm, tail bud mesoderm.