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Fig. 1. An immobile pivotal domain in the caudal midbrain. (A,B) Posterior, (C) posterolateral, and (D) lateral views of the dissected (A-C) or whole-mount (D) neural tubes of HH19-21 chick embryos that received a graft or DiI crystals as schematized on the left. Small lateral grafts (A) and lateral DiI crystals (B) all converged towards the midline. They avoided a small medial circular area in the caudal midbrain (arrowheads in A,B). Small grafts (C) or DiI crystals (D) inserted into the center of the caudal mesencephalic vesicle apparently remained still (arrowheads in C and D) and ended in a protrusion or `beak' that marks the caudal midline of the mesencephalon. This is best seen on lateral views of the dissected neural tube (E,F arrowheads).