Fig. 4. Region B is required to maintain the anterior somitic boundary after 9.0 dpc. (A-F) X-gal staining of transgenic embryos. (A,B) Expression patterns for construct Cb4Z (A) and CBb4Z (B) were identical at 8.5 dpc. Both had an anterior boundary at so6/7 in the paraxial mesoderm. (C,D) Anterior boundaries of expression remained identical at 9.0 dpc. (E,F) By 9.5 dpc, the somitic boundary of Cb4Z expression shifted one segment posteriorly (E), whereas that of construct CBb4Z remains at so6/7 (F). Black arrowheads indicate the position of so7. (G-J) X-gal stained transgenic embryos counterstained with acid fuchsin. (G,H) At 9.0 dpc, two somites could be detected between the anterior boundaries of X-gal staining in the paraxial mesoderm and neural tube (black arrows) for constructs Cb4Z and CBb4Z. (I,J) By 9.5 dpc, the gap had extended to three somites with construct Cb4Z (I) but remained at two with CBb4Z (J), confirming the timing of the posterior shift of the expression boundary.

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