Fig. 1. Patterns of gene expression in rhombomeres and cranial motoneurones. (A) Diagram of a flat-mounted stage 28 chick embryo hindbrain and caudal half of the midbrain showing the different combinations of Lim homeobox genes expressed by each motor nucleus and the expression pattern of three Hox genes, Hoxa3, Hoxb3 and Hoxd4. Rhombomere boundaries are no longer visible at this stage but are shown for clarity. Motor nuclei: III, oculomotor; IV, trochlear; V, trigeminal; VI, abducens; VII, facial; IX, glossopharyngeal; X, vagus; XI, accessory; XII, hypoglossal [adapted, with permission, from Varela-Echavarría et al. (Varela-Echavarría et al., 1996)]. (B-F) Flat mounts of E6 chick midbrain and hindbrain preparations. Red lines indicate the midbrain-hindbrain boundary. Whole-mount in situ hybridisation for Islet2 (B), Hb9 (C), Hoxa3 (D), Hoxb3/Islet2 (E) and Hoxd4/Islet2 (F). mb, midbrain; hb, hindbrain; r, rhombomere; fp, floor plate; a, accessory; cva, contralateral vestibuloacoustic. Scale bar: 500 µm.