The origin and development of the nervous system is a gradual process of cell division, migration, and specialization. The establishment of neural circuits requires cell-to-cell recognition. Despite an unceasing search, the mechanisms accounting for this cell-to-cell recognition remain unknown. In contrast, the stages in prenatal development from conception to birth and the sequence of events in the formation of the nervous system are known in considerable detail. The major purpose of Part 1 is to review the ontogeny of the spinal nervous system, with emphasis on the continuous remodeling phenomena that occur as a result of changes in neuronal activity or in the biochemical milieu. The underlying rationale for focusing on the details of prenatal maturation is to identify and analyze cell-to-cell interactions and to define their critical periods. This type of information is expected to provide explanations for previously unexplained developmental phenomena, to improve ability to diagnose and prognosticate in newborns with congenital anomalies of the nervous system, and to provide therapists with insights for improving treatment techniques for neonates with neurological deficits.