Abstract Cerebral neocortical explants derived from 2 or 3 day old mice demonstrate a remarkable degree of neuronal and architectural maturation after several weeks in vitro. Pyramidal neurons contain apical dendrites with oblique branches and well formed branched basal dendrites with dendritic spines. A laminar organization of both neurons and fibers is readily apparent. Neurites are organized into four major bundles, including tangential, submarginal, intermediate, and deep bands. Many cultures also contain a rich plexus of subcortical fibers. The pattern of organization which develops in vitro is unique to cultures of cerebral neocortex, and it serves to distinguish them from cultures of spinal cord, cerebellum and hippocampus, all of which have their own characteristics architectural features. Tissue culture studies indicate that isolated fragments of nervous tissue have a high degree of intrinsic developmental capability. These observations suggest that some aspects of structural development of the nervous system may be rather rigidly determined.