[Changes in the retino-tectal system of the turtle Testudo horsfieldi following enucleation]

Studies have been made on.morpho-functional changes in the optic nerve and midbrain tectum after enucleation of the tortoise T. horsfieldi. It was found that intensive degeneration of terminals which follows "dark" type (3--31/2 months after enucleation) corresponds to degeneration of myelinated fibers. Terminals degenerating accordingly to "light" type (2--3 months) presumably belong to unmyelinated fibers, which degenerate at this period in the nerve. Terminals which follow "vesicular" degeneration, survive up to 6--61/2 months, probably due to survival of some unmyelinated fibers in the nerve. During degeneration of the optic fibers, corresponding changes in amplitude-temporal parameters of the evoked potentials in the midbrain tectum were observed during electrical stimulation of the optic nerve. These changes include the increase of the interval between the stimulus and the peak of both components, as well as the decrease of the amplitude of the responses. These changes were originally observed 11/2--2 months after enucleation, becoming more evident at later stages; 6--61/2 months after enucleation, both components of the evoked potential disappeared almost simultaneously.