The embryonal central nervous system (CNS) neoplasms are reviewed with special reference to their differentiating potential and in the light of current neuro-oncogenetic concepts partly derived from the experimental induction of neural tumors. The conceptual (and, occasionally, practical) distinction between adult-type and embryonal CNS tumors raises a complex problem, because neoplastic transformation essentially involves replicating stem cells in tissues of renewal and because in the human brain such cells are found mostly in the course of CNS development. A cytogenetic scheme is therefore needed to serve as a frame of reference for a classification of embryonal CNS tumors that will account for the different histological entities documented so far and for the range and the restrictions of their differentiating capabilities. Most embryonal CNS tumors can be fitted into such a scheme. The cerebral medulloepithelioma, the cerebral and cerebellar neuroblastomas, the primitive polar spongioblastoma, and the ependymoblastoma show characteristic morphological features and a correspondingly distinctive cellular differentiating potential. The differentiating capabilities of the cerebellar medulloblastoma, the pineoblastoma, and the retinoblastoma are also distinctive, and are apparently determined by the cytogenesis of the area of the CNS in which the tumors originate. The indiscriminate application of a simplistic concept that would include all the so-called "primitive neuroectodermal tumors" into a single neuroepithelial tumor entity is unlikely to bring further understanding to the problem.