Fig. 1. Schematic organization of the MH domain at the 10-somite stage (A,C) and at 24 hpf (B). All views are anterior towards the left; A and C are dorsal and ventral views of the alar and basal plates, respectively; B is a sagittal view, the broken line delimiting the alar/basal boundary. The early MH domain comprises the mes- and metencephalic vesicles; the contribution of each vesicle to the late MH derivatives, as demonstrated in transplantation experiments in the avian embryo (Hallonet and Le Douarin, 1990; Hallonet et al., 1993; Martinez and Alvarado-Mallart, 1989) (and without considering the floor and roof plates) is color-coded and indicated by the vertical lines: (1) the alar plate of the mesencephalic vesicle contributes to the tectum; (2) in addition, the caudal third of the mesencephalic vesicle is at the origin of the alar part of the isthmus and dorsomedial part of the cerebellar plate (future vermis) and alar part of r2; (3) the alar plate of the metencephalon gives rise to the lateral cerebellum (future hemispheres); (4) the basal plate of the mesencephalic vesicle gives rise to the tegmentum; (5) the basal plate of the metencephalic vesicle gives rise to the pons (basal r1) and basal plate of r2. The isthmus is colored in yellow. Its basal part has not been precisely mapped and was not studied for its inductive properties of MH fate; it is drawn here based on the expression pattern of isthmic organizer markers such as wnt1 and fgf8. The `intervening zone' is defined as the territory delayed in neurogenesis (Geling et al., 2003). It is located at the MHB but its spatial relationship with the isthmus has not been established. Cb, cerebellum; Di, diencephalon; Is, isthmus; IZ, intervening zone; Mes, mesencephalon; Met, metencephalon; Myel, myelencephalon; Po, pons; r, rhombomere; Tc, tectum opticum; Tg, tegmentum.