Fibre order in the normal Xenopus optic tract, near the chiasma

In juvenile Xenopus retinotopic fibre order in the optic tract near the chiasma was investigated by labelling small groups of optic fibres from peripheral retina with HRP. This selective fibre labelling with HRP was combined with autoradiography following administration of tritiated thymidine to the eye, so that the HRP-labelled fibres could be located within the borders of the optic tract. Fibres arising from the periphery of all four retinal quadrants were superficially located in the optic tract near the chiasma, with dorsal retinal fibres showing the greatest tendency to travel deep in the diencephalon. Retinal lesions closer to the optic nerve head labelled fibres which ran deeper in the optic tract. Near the chiasma, fibres from ventral retina tended to group rostrally while fibres from dorsal retina tended to group caudally. However, no obvious localization of fibres arising in temporal or nasal retina was seen in the lower optic tract.