The development of the ipsilateral retinothalamic projections in the Xenopus toad.

The appearance of ipsilateral terminal degeneration in the thalamic visual centers is studied with the aid of the Fink-Heimer II technique in Xenopus larvae and postmetamorphic toadlets following unilateral eye removal. The youngest age at which degeneration can reliably be shown in the lateral geniculate complex (nucleus of Bellonci and corpus geniculatum thalami) was at stage 62, that is, 10-12 days before metamorphic climax. The whole compliment of the ipsilateral retinothalamic projection develops only in postmetamorphic toadlets, and it is much more abundant in Xenopus than in Rana species. The first appearance of the ipsilateral degeneration in the lateral geniculate complex coincides in time with the first ipsilateral visuotectal responses described by others. The possibility that the two phenomena may have a closer relationship than the mere time coincidence, is discussed.