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

The retinofugal projections of the chameleon were investigated by mean s of autoradiography, horseradish peroxidase and fluorescent technique s 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 t o 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, late ral part of nucleus dorsolateralis anterior; and nucleus ventrobasalis ), four pretectal nuclei (nucleus griseus tectalis, nucleus lentiformi s mesencephali, nucleus geniculatus pretectalis and nucleus posterodor salis), the optic tectum (stratum griseum et fibrosum superficiale) an d the tegmental nucleus opticus tegmenti. Our findings are, in general , compatible with previous descriptions of the primary visual system i n other species of lizards. However, they indicate three features part icular to chameleons: first, the total absence of an ipsilateral retin ofugal 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 respec t to other lizards.