Fig. 2. Midline crossing and onset of nucleogenesis in the myelencephalon. (A) EYFP-labelled cells migrated tangentially towards the ventral midline (arrow) from the lRL (asterisk). (B) EYFP-labelled cells in the dorsal myelencephalon on the side ipsilateral to the injection. Labelled neurons that had left the lRL migrated along the dorsal margin (arrowheads). (C) EYFP-labelled neurons found near the ventral midline. The inset shows a higher-power view of the area outlined by the rectangle in C. Many labelled cells cross the ventral midline (arrowheads in the inset). Broken line indicates the midline. (D) A higher-power view of the area outlined by the rectangle in A. EYFP-labelled neurons do not form an aggregate ipsilaterally, although cells that appear to originate from the contralateral side as judged by Mbh2 expression do so. A-D are from E14.5 animals. (E-H) The direction of the EYFP-labelled cell migration in the regions of ECN (E) and LRN (G) changed from tangential to radial, with F and H showing higher-magnification views of the areas outlined in E and G, respectively. At E15.5, some of the labelled cells had begun to migrate radially (F,H), extending leading processes (arrowheads in F and H), and had aggregated within the ECN (arrow in E) and LRN (arrow in G). In each panel, EYFP was introduced into the left side. Dorsal is upwards. Scale bar: 200 µm for A,E,G; 50 µm for C,D; 25 µm for the inset in C; 20 µm for B,F,H.