The bipolar defects observed in schizencephalies-clefts in the hemispheric mantle on the one hand, absent septum pellucidum on the other--without any anatomic or functional continuity suggest that there is some sort of common specific vulnerability of both structures. A study of the correlation between lobar location of the clefts and involvement of the septum pellucidum was undertaken, considering the hypothesis that the septum pellucidum is the portion of a "medial medullary velum" that corresponds to the frontal lobe, while the psalterium would correspond to the parieto-occipital lobe and the fimbria corresponds to the temporal lobe. This retrospective study of 16 cases of schizencephaly properly investigated by MR discloses a perfect correlation, all cases with absent septum pellucidum having clefts into the frontal lobe, all cases with present septum pellucidum having clefts in the parietal, temporal, and occipital lobes, and only the few instances (3 cases) of overlapping findings being characterized by clefts in the central area, where the distinction between posterior frontal and anterior parietal lobes is uncertain because of the cortical dysplasia related to the clefts. Partial defects of the septum also proved to correlate closely, topographically, with the location of the clefts. Therefore, the facts confirm a segmental organization of the mantle and septal defects, suggesting a developmental rather than a destructive mechanism, which could at least be related to a segmental pattern of selective vulnerability.