A series of 10 central nervous system tumors thought to be composed of neurons and their multipotential precursors, are described and analyzed. Three are interpreted as hamartomas, seven as neoplasms. Six of the latter contained mature, well formed neurons, and of these, four contained anaplastic neurons, and two, and possibly a third, contained astrocytomatous tissues, as well. One neoplasm contained anaplastic neurons in the absence of well differentiated neurons. The observations suggest that these are embryonal tumors arising in nests of primitive multipotential neurectodermal cells which may mature and differentiate into neurons and neurologia, these being normal, malformed or neoplastic. Should neoplastic change supervene at any stage of maturation, either or both of two distinct processes may then operate to alter the appearance and biological activity of the tumor: the cells may continue to mature and differentiate, and grow more slowly, and the cells may become more anaplastic and grow more rapidly. Dedifferentiation, the concept that mature cells pass backward along the normal path of embryologic development to become immature cells, is thought not to be involved; if it occurred, the malignant neuronal tumors would tend to resemble neuroblastomas like those of the adrenal, but this is not the case. Much fibrous connective tissue is present in all of the neoplasms, and this is thought to be a product of Schwann cells formed from primitive reticular cells in response to the neuronal fibers present in such tumors, or which had been present prior to the onset of anaplastic changes.