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Fig. 7. Induction of gene expression by planar signals. Expression in tailbud stage embryos is shown on the left column (anterior to the left) and expression in age-matched Keller sandwiches is shown on the right column (anterior to the top). (A,B) X-dll3; (C,D) N- CAM; (E,F) En-2; (G,H) X-twi; (I,J) Actin. In A and B, arrows point to expression in the forebrain and arrowheads to expression in the cement gland. A higher magnification of A (Fig. 4G) shows that the level of X-dll3 expression in the prosencephalon is comparable to the level in cement gland and olfactory placode. Note that expression of X-dll3 in a Keller sandwich occurs around the edge of the non-elongated part of the neural ectoderm. In D and J, the limits of the mesoderm are denoted by arrowheads, illustrating that in a Keller sandwich the mesoderm forms away from the neural ectoderm. In D, the anterior limit of N-CAM hybridization is marked by an arrow and roughly the same level has been marked by arrows in B, F and H. In H, note that the X-twi hybridization is localized close to the base of the non-elongated part of the neural ectoderm (arrowheads), at about the same A-P level with en-2 (arrowheads in F). Both markers are clearly more posterior than X-dll3 (compare B with F and H). The staining denoted by an asterisk in F is an artifact. |
