Fig. 3. Mouse rhombomeres are well integrated in the chick hindbrain. (A) Schematic of a homotopic r5-r6 grafting from an E8.5 Rosa-26 lacZ transgenic mouse donor into a HH stage10 chick host. (B,D) Dorsal views of flat-mounted chimaeras after X-Gal staining at the stages indicated. In (B) a few LacZ-positive cells spread into r4 (arrow) and the posterior boundary of the mouse graft is prominent (arrowhead). In (D) the chick r4/mouse r5 boundary is totally regenerated (see arrow) and X-Gal staining is exclusively restricted to the mouse graft. (C) Dorsal view of a flat-mounted chimaeric hindbrain hybridised with mouse kreisler. mr5/6, mouse graft consisting of r5 and r6; fp, floor plate.