The distribution of fibres in the optic tract after contralateral translocation of an eye in Xenopus.

In Xenopus embryos of stage 30 the right eye was translocated, without rotation, to a left host orbit. Shortly after metamorphosis the visuotectal projection through the operated eye was mapped electrophysiologically and shown to be normal dorsoventrally but reversed nasotemporally. Labelling of small groups of retinal axons with HRP showed that the fibre trajectories from dorsal and ventral retina were normal, whereas fibres from nasally placed retina had diencephalic pathways and tectal terminations typical and temporal fibres, and fibres from temporally placed retina had diencephalic pathways and tectal terminations typical of nasal fibres. Thus from just beyond the chiasma the fibres had already achieved the major uniaxial rearrangement necessary to establish a normal tract distribution despite the eye translocation. The fibre rearrangement required to permit the formation of a nasotemporally inverted visuotectal projection appears, therefore, to occur not on the tectum or in the optic tract, but either within the nerve or at the chiasma.