There is evidence that oligodendrocytes in the spinal cord are derived from a restricted part of the ventricular zone near the floor plate. An alternative view is that oligodendrocytes are generated from all parts of the ventricular zone. We reinvestigated glial origins by constructing chick-quail chimeras in which dorsal or ventral segments of the embryonic chick neural tube were replaced with equivalent segments of quail neural tube. Ventral grafts gave rise to both oligodendrocytes and astrocytes. In contrast, dorsal grafts produced astrocytes but not oligodendrocytes. In mixed cultures of ventral and dorsal cells, only ventral cells generated oligodendrocytes, whereas both ventral and dorsal cells generated astrocytes. Therefore, oligodendrocytes are derived specifically from ventral neuroepithelium, and astrocytes from both dorsal and ventral.