Retino-retinal projections in three anuran species.

In juvenile and adult Xenopus laevis, in adult Bufo marinus and Rana esculenta frogs retino-retinal projections were traced by filling the central stump of one optic nerve, cut 2-3 mm from the eye, with horse-radish peroxidase (HRP) or cobaltic-lysine complex (CLC). The presence of retino-retinal projections was confirmed in all 3 species both in the juvenile and the adult. Up to 12 ganglion cells per retina were found to be filled retrogradely with HRP together with optic axons filled anterogradely with CLC. These findings suggest that (1) a small proportion of ganglion cells project, erroneously, to the opposite retina and (2) this erroneous retino-retinal projection persists throughout the whole lifespan of the animals.