Fig. 6. Robo-positive axons cross the CNS midline in kra mutant embryos. (A-F) Stage 16 embryos carrying the apC-tau-lacZ marker were doubly stained with anti-Slit (A,D) and anti-ß-galactosidase (B,E). (A-C) A wild-type embryo. (D-F) A kra¹/kra² mutant embryo; some Ap axons ectopically cross the midline (arrowheads in E,F). (G-L) Stage 16 embryos carrying the apC-tau-lacZ marker were doubly stained with anti-Robo (G,J) and anti-ß-galactosidase (H,K). (G-I) A wild-type embryo. (J-L) A kra¹/kra² mutant embryo; Robo is abnormally detected in the commissural tracts that contain Ap axons crossing the midline (arrowheads in K,L). Scale bar: 10 µm.