An isthmo-optic system in a bony fish.

This investigation reveals the existence of an isthmo-optic system in a bony fish for the first time. Two cell aggregates of the isthmic region project bilaterally to each eye in Polypterus. The crossed connections are significantly more developed than the uncrossed ones. These findings provide further evidence for the presence of bilaterally projecting isthmo-optic systems in early stages of vertebrate evolution. Furthermore, they suggest that a loss of one or all of these connections during evolutionary or ontogenetic development reflects a parcellation process as proposed by the parcellation theory.

Optic tract cells projecting to the retina in the teleost, Pantodon buchholzi.

Horseradish peroxidase was employed to trace retino-fugal and retino-petal connections in the teleost fish, Pantodon buchholzi. Most of the reciprocal connections found were within the range also observed in previously studied species of teleosts. Of particular interest is the discovery of cells located within the optic tract and projecting to the retina. These neurons were investigated electron microscopically.