PROJECTION OF OPTIC FIBERS TO VISUAL CENTERS IN A TURTLE (EMYS ORBICULARIS)

Studies of retinal projections to the thalamus and midbrain of the turtle were based on a personal modification of the Nauta-Laidlaw technique (modified Nauta method) after unilateral enucleation. Decussation of optic fibers in the chiasma is incomplete. In the thalamus, optic fibers are found to terminate in three nuclei-with greater density in the corpus geniculatum laterale (lateral geniculate body) (LGB) and more sparsely in the nucleus suprapeduncular and nucleus ovalis. Retinal projections to the LGB assume a focal pattern, being somewhat more compact in the lateral neuropil region. Optic fibers are also shown to end in a group of pretectal nuclei: n. [nucleus] pretectalis dorsalis, n. lentiformis mesencephalis, n. comissurae posterior. In addition, terminations of optic fibers have been revealed in the three upper layers of the tectum. Peculiarities of preterminal and terminal degeneration of retinal fibers have been distingushed in the different tectal layers. In the second stratum, terminations of large fibers are mostly seen with a characteristic appearance of lumpy pericellular (preterminal) degeneration. In the third stratum, both large and finers degenerate, showing fine debris of preterminal degeneration. Different patterns of terminal degeneration have been revealed in tectum and thalamus. The maximal size of optic fibers in the tectum proves to be larger than in thalamus. Available evidence is discussed with particular reference to comparison of the phylogenetically more recent retinothalamic system and more ancient retinotectal system.