pax-6 is expressed in limited regions of the neural keel. Paraffin sections of embryos were hybridized with 35S-labelled RNA probes (probe A, probe C gave the same results). (A,B) Parasagittal sections of a 14 h embryo. Expression is seen in the optic vesicle (A, B), the prospective forebrain (B) and the prospective hindbrain and spinal cord (A, B). Lines in B indicate the plane of section in panels C and D. (C) Cross-section of the prospective forebrain at 14 h. Expression is seen in the dorsal half, excluding the dorsal-most cells of the forebrain. (D) Cross-section of the neural keel at 14 h. Expression is seen in the ventral half. (E) Parasagittal section of the optic vesicle (ov) at 14 h. Expression is seen in the ventral and lateral part of the vesicle. Little or no expression is detectable in the dorsal part. Expression is also seen in the ectoderm (ect) overlying the optic vesicle. (F) Horizontal section of an 18 h embryo. Expression is seen in the diencephalon (di) and the myelencephalon (my). (G) Cross-section through the trunk of a 24 h fish. Expression is detectable in the ventral half of the neural tube, excluding the floorplate and cells adjacent to it as was confirmed by inspecting the corresponding bright-field images (not shown). Anterior is to the left in A, B, E and F and dorsal to the top in C, D and G. Scale bar is 200 μm in A, B, 160 μm in C, D, G and 120 μm in E, F. ce: cerebellum.