Fig. 7. Barrier and removal experiments. (A) Schematic drawings illustrate the use of a barrier immediately to the right (top) or left (bottom) of the neural tube in a stage 7 embryo. (B) Picture of an embryo immediately after the operation. (C) Normal Nodal expression after this experimental manipulation. (D) Ectopic Nodal expression induced by a BMP-soaked bead on the right after the placement of the barrier. (E) A double hybridization for Nodal (blue) and Lefty1 (yellow) shows ectopic Nodal expression on the right induced by the BMP-bead but Lefty1 appears unaffected. (F) Schematic drawings illustrate the removal of the left LPM. (G) Stage 6 embryo immediately after the removal of the left LPM. (H) Nodal expression in an embryo 5 hours after the operation. (I) An embryo subjected to removal of the left LPM at stage 6 and implanted with a BMP-soaked bead on the right hybridized for Nodal and Lefty1.

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