The acquisition of tectal positional specification in Xenopus

During early embryogenesis before the outgrowth of the optic fibres the left eye blastema was removed. At the time of metamorphosis the right tectum, which has thus not received an optic fibre input, was surgically rotated by 180 degrees and the right optic nerve deflected to the rotated tectum. Between two and five months after the last operation the retinotectal projection was mapped electrophysiologically and autoradiographically. The visuotectal projection was found to be rotated, indicating that the ingrowing optic fibres had reached the same tectal areas that they would have reached had the tectum not been rotated. Since no retinal input reached the rotated tectum until after metamorphosis, it is concluded that tectal positional markers which serve as targets for the deployment of retinal fibres are inherent to the tectum and are generated independently of optic fibre input, presumably during early larval life.