Fig. 4. Chick motoneurones migrate into mouse r5/6 in mouse-chick chimaeras. (A,E,G) Homotopic mouse-chick grafts. (B-D,F,H) Dorsal views of flat-mounted hindbrain after in situ hybridisation with (B,C) chick Isl1 riboprobe (in blue) and B2 mouse-specific probe (B, in red), and (D,F,H) chick Hoxb1 (in blue). (B) At HH stage 20, a compact group of chick Isl1-positive cells (see arrow) has entered the anterior border of mouse r5, whereas at HH stage 24 (C) a prominent stream of chick Isl1-positive cells has invaded the mouse tissue (surrounded by red dots). The asterisk indicates that a subpopulation of r4 neurones migrates laterally. (D) A r4-specific population positive for chick Hoxb1 expression runs along the mouse floor plate included in the graft. (F) The sole presence of mouse r5 induces migration of chick Hoxb1-positive cells into the mouse graft. The inset in F shows that thick borders surround the grafted mouse r5 and includes chick Hoxb1-positive cells. (H) The right side shows a ventral protrusion of chick Hoxb1 expression through chick r5 towards the mouse tissue (labelled with a red fluorescent cell tracker). Red dots indicate the external margin of the mouse grafts.