The case is that of a female fetus of 17 to 18 weeks' gestation with major defects of the central nervous system: (1) The thoracic vertebrae demonstrated rachischisis, with segmental diplomyelia; the duplicated cords were dissimilar in size and lay side by side within a single meningeal sheath lacking a dividing septum or spur. Cranially to the divided cord lay an unsplit segment of "open cord" lacking the posterior elements and exposing the centrally placed ependyma of the central canal flanked by glial and epidermal lining, respectively; it could be regarded as an example of a meningomyelocele. (2) Heterotopic massed ependymal cells, some of which were actively proliferating, were associated with the choroid plexus in the brain. Minor anomalies included cerebellar heterotopia and the malpositioning of dorsal root ganglia outside the meningeal sheath. Because the ependyma is such a powerful inducer of the development of neighboring tissue, the findings could be united by a common pathogenic theme, viz problematic ependymal development and migration within both the brain and spinal cord. The causative agent responsible for these abnormalities remains unidentified, but the balance of evidence suggests that its effect was felt during the second week of postconceptual age.