The mechanisms leading to cell diversification in the Vertebrate central nervous system are still poorly understood. We have analyzed neural differentiation potentialities of the embryonic chick optic nerve. In the adult, the optic nerve is made up of astrocytes and oligodendrocytes ensheathing retinal axons, but it is entirely devoid of neuronal cell bodies. Using explant cultures and specific cell type markers, we demonstrate that in fact the embryonic optic nerve contains cells endowed with neuronal potentialities but is initially devoid of a potential for oligodendrogenesis. Studies by other groups in rodents suggest that oligodendrocyte precursors may be initially restricted to the ventral region of the developing spinal cord. Taken together, these results indicate that early in development, oligodendrocyte precursors are not distributed homogeneously in the neuroepithelium. Preliminary results in our laboratory show that the specification of the oligodendrocyte lineage in the chick spinal cord may depend on ventral signals from the notochord.