The anti-retinotopic organization of the frog's optic nerve

In Rana pipiens, axons marked by the intraretinal application of horseradish peroxidase (HRP) were traced within the optic nerve and tract. Axons arising from dorsal regions of the peripheral retina collect at the dorsal end of the elongate optic disc and form a compact group on the dorsal side of the nerve. Correspondingly, ventral axons locate on the ventral side of the nerve. However, nasal and temporal peripheral axons share passage on both the nasal and temporal sides of the nerve, segregating only upon reaching the brain. The ultimate sorting of nasal and temporal axons in the brain, following their intermingling in the optic nerve, supports the operation of a chemoaffinity mechanism, rather than passive mechanical guidance.