The first extending axons in a pathway interact closely with immature glia cells. In the forming corpus callosum of vertebrates, primitive glia cells first migrate to the ependymal zones of hemispheres and the dorsal septum wall to form a transient structure that the pioneer axons of the callosal fibers use to extend. The signaling between glia and neurons in the developing nervous system is reciprocal. For instance, in the fly visual system, axons of photoreceptors require glia to exit the eye stalk whereas glia cells rely on signals from neurons to migrate back along axons.