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.