Fig. 4. The mouse ventral midbrain has a medial arc expressing molecular markers of the OMC and the pRN. (A) Emx2 labeling identifies arcs in mouse E12.5 midbrain wholemounts, oriented as in Fig. 1. (B,C) Isl1 (B) and Brn3a (C) labeling of the OMC and the pRN in the mouse medial arc. (D) Combined Isl1 and Emx2 labeling confirms that the organization of the mouse medial arc is similar to that of the chick. IS, isthmus; OMC, oculomotor complex; rFP, rostral, or midbrain, floor plate; RN, red nucleus.