Thalamotelencephalic projections in the turtle (<i>Pseudemys scripta</i>)

Abstract The telencephalic projections of the turtle thalamus were studied using the Fink‐Heimer ('67) technique for staining degenerated axons and their terminals. Large thalamic lesions produced terminal degeneration in the basal telencephalic nuclei, the core of the dorsal ventricular ridge and the outer half of layer I in general cortex. A variety of control lesions confirmed that these projections arise in the thalamus. Circumscribed thalamic lesions revealed first, that there is some degree of spatial organization in the turtle's thalamocortical projection system and second, that at least one sensory relay nucleus, the dorsal lateral geniculate, projects to general cortex. Detailed comparisons of the turtle's thalamotelencephalic projections with those present in two primitive mammalian species, the hedgehog and the opossum, provided a basis for identifying probable homologies in the forebrains of reptiles and mammals.