Depolarization shortens the optic projection.A–D, Representative examples of stage 40 whole-mount brain preparations showing the HRP-labeled optic projection in a control brain (A) and when 1 μm TTX (B) or 20 mm KCl (C, D) are applied to the exposed brain. E, Graph showing the optic tract length measured in dehydrated brains. Tract length was measured in normalized BRUs and then converted to micrometers (1 BRU, ∼620 μm; Chien et al., 1993). The optic tract is unaffected by TTX application, but is significantly shorter in brains exposed to depolarizing conditions of external K (**p < 0.01). Tec, Tectum;Pi, pineal; Hyp, hypothalamus;Di, diencephalon; Tel, telencephalon;arrowhead, midbrain–hindbrain isthmus;Oc, optic chiasm; ot, optic tract.White dots (A) show the approximate border of the anterior tectum. Scale bar (shown inD), 100 μm.