LacZ staining in the central nervous system of heterozygous (SCL^lacZ/w) embryos. (A and B) At E11.5, lacZ staining is apparent in the presumptive motor neurons in the ventral neural tube (arrows). (C) Lateral and (D) dorsal views at E12.5 and (E) lateral and (F) dorsal views at E13.5, showing expression of lacZ in the developing midbrain. At E18.5 (G) and (H) and in the adult mouse (I and J), lacZ expression is strongest in the superior colliculi, but lower levels persist in the inferior colliculi. Weak lacZ expression is evident in tracts on the ventral surface of the hindbrain (H) and (J).