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Fig. 4. Expression of Wnt/ß-catenin reporter transgenes in vivo is Wnt3a dependent. (A) A five-somite TOPgal embryo showed strong expression in the node (arrow), primitive streak (ps) and posterior mesoderm. (B) Cross-section through the node of embryo shown in A. (C,E) A head-fold stage BATlacZ embryo showing ß-gal expression in the primitive streak (for posterior view, see inset), anterior psm (black arrow) and the node (curved white line). (D,F) BATlacZ expression was reduced in the Wnt3a-/- streak, and was not expressed in the node (curved white line) and anterior psm (black arrow). (G) ß-Gal activity in six-somite BATlacZ embryo visualized with Salmon-gal (red). (H) lacZ mRNA expression in a five-somite BATlacZ embryo visualized by whole-mount in situ hybridization. (A, and insets in C,D) Ventroposterior views. (E,F) Ventral views, left side of the embryo is on the left. (C,D,G,H) Lateral views, anterior is towards the left.