AN EXPERIMENTAL REEVALUATION OF THE PRIMARY VISUAL-SYSTEM OF THE EUROPEAN CHAMELEON, CHAMAELEO CHAMELEON

The retinofugal projections of the chameleon were investigated by means of autoradiography, horseradish peroxidase and fluorescent techniques after intraocular injection of tracers. An ipsilateral contingent of visual fibers and projections is absent. The retinal fibers decussate completely in alternating fascicles at the optic chiasma and course to terminate in two hypothalamic nuclei (nucleus suprachiasmaticus and nucleus opticus periventricularis hypothalami posterior), six thalamic nuclei (nucleus ovalis, nucleus geniculatus lateralis dorsalis partes lateralis and medialis, nucleus geniculatus lateralis ventralis, lateral part of nucleus dorsolateralis anterior; and nucleus ventrobasalis), four pretectal nuclei (nucleus griseus tectalis, nucleus lentiformis mesencephali, nucleus geniculatus pretectalis and nucleus posterodorsalis), the optic tectum (stratum griseum et fibrosum superficiale) and the tegmental nucleus opticus tegmenti. Our findings are, in general, compatible with previous descriptions of the primary visual system in other species of lizards. However, they indicate three features particular to chameleons: first, the total absence of an ipsilateral retinofugal projection; second, the existence of an additional hypothalamic visual center located in the posterior mediodorsal hypothalamus; and third, the large size of nucleus opticus tegmenti. These features are discussed in terms of the taxonomic position of chameleons with respect to other lizards.