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suMMARY The connections of an ectopic midline spinal ganglion associated with an asymptomatic sacral diastematomyelia were studied. The ganglion was intercalated in the ventral root of one hemicord and sent its efferents to the dorsal root of the other hemicord. The afferents joined the anterior root to form a midline intradural spinal nerve in the cauda equina. Islands of ectopic glia were present in both roots and the spinal nerve. Both the midline position of the ganglion and the glial heterotopias can be tentatively explained by the failure of incorporation of the dorsal cell wedge ("Zwischenstrang") into the divided neural tube.While isolated ectopic neurons are not uncommon in the spinal roots of normal subjects,' intradural ectopias of fully formed spinal ganglia are almost invariably associated with dysraphic malformations.2 An interesting variant, associated with diastematomyelia, is that in which an ectopic spinal ganglion occupies a midline position dorsal to the cleft separating the two hemicords. A case of this type was described by Gagel3 also illustrated by Ostertag4 and similar ones are recorded in subsequent publications.2 5 Little is known, however, of the connections of the ectopic ganglia or of the structure of the adjacent spinal roots. In reporting a further example of this malformation, an incidental finding in an asymptomatic adult, we describe the anatomical relationships of the ectopic ganglion and to review the pathogenesis of this lesion in light of current embryological concepts. Case reportThe patient was a 63 year old hypertensive diabetic who died ofheart failure. The 1480 gram brain was unremarkable. The spinal column showed no abnormality. The spinal cord was exposed anteriorly. The dura was single and intact. The