The ON and its myelinated axons both shorten and transiently widen at metamorphosis. (A) Head remodeling in X. laevis at premetamorphic NF stage 58, metamorphic climax stage 62, and immediately postmetamorphic climax stage 66. (Scale bar: 1 cm.) ON length (B) and cross-sectional area (C) during metamorphosis. F, juvenile frogs 4 wk postmetamorphosis. (D) Myelinated axon axoplasm cross-sectional area, averaged per ON. Values derive from 6,975, 5,759, 5,267, and 6,076 axons at stages 58, 62, and 66 and in F, respectively. (E) Representative traced TEM micrographs of the ON cross-sections at stages 58 and 66. Myelin sheaths (blue), axoplasm (yellow), focal periaxonal space enlargements (white and arrowheads), and myelin not associated with axons (cyan) are illustrated. (Scale bar: 2 μm.) Mean ± SD is shown (n = 6 ON per stage). *P < 0.05; **P < 0.01; ***P < 0.001 (by the Games–Howell test).