Primary retinal targets in the Atlantic loggerhead sea turtle, Caretta caretta.

Autoradiographic analysis distinguished twelve primary retinal targets in the diencephalon and the mesencephalon of the Atlantic loggerhead sea turtle, Caretta caretta. While the majority of fibers terminate contralaterally, sparse labelling is seen over ipsilateral thalamic nuclei. The dorsal optic nucleus is the most expansive retinal target in the dorsal thalamus. Four nuclei ventral and one dorsal, to the dorsal optic nucleus, receive retinal input. Before terminating in the optic tectum, labelled fibers pass through the pretectum terminating in four nuclei. Within the superficial zone of the optic tectum, three terminal zones are recognized. A distinct accessory tegmental tract separates from the main optic tract terminating in the basal optic nucleus. While such a multiplicity of retinal targets occurs among other reptiles, birds and mammals, it is presently impossible to accurately recognize visual homologies among amniotic vertebrates.