Visual projections in larval Ichthyophis kohtaoensis (Amphibia: gymnophiona)

The visual projection patterns of retinal efferents were studied in larval Ichthyophis kohtaoensis by means of anterogradely transported HRP. Our results show in all larvae a projection contralateral to a thalamic terminal field, a pretectal terminal field, and a basal optic neuropil, but only a sparse innervation of the contralateral tectum. In addition, all larvae possess an uncrossed projection to a thalamic and a pretectal terminal field. The fibers are bilaterally almost confined to the medial optic tract with only a few fibers running in the marginal and basal optic tract. The ipsilateral and contralateral tracts and terminal fields seem to enlarge during larval life. Comparison with other amphibian orders reveals that larval Ichthyophis are unique in that they develop the medial optic tract and the related thalamic and pretectal terminal fields very early in larval life. In addition they possess only a very sparse tectal projection, though it is the largest projection in larval urodeles and anurans. This suggests a selective phylogenetic loss of those ganglion cells or collaterals which project mainly to the tectum in other amphibian orders and a change in the ontogenetic program leading to an earlier development of the medial optic tract in Ichthyophis as compared to urodeles and anurans.