GETTING THERE AND BEING THERE IN THE CEREBRAL-CORTEX

The mammalian neocortex is composed of functional areas that are speci fied to process particular aspects of information. How is this specifi cation achieved during development? Since cells migrate to their final positions in the developing nervous system, a central issue is the re lation between cellular migration and positional information. This rev iew combines evidence for early positional specification in the develo ping cortex with evidence for cellular dispersion during migration. A model is suggested whereby stable cues provide positional information and minorities of 'displaced' cells are respecified accordingly. Compa rison with other parts of the CNS reveals that cellular dispersal is u biquitous and has to be included in any mechanism relaying positional specification. Ontogenetic and phylogenetic considerations suggest tha t radial glial cells might provide the positional information in the d eveloping nervous system.