Transitional forms of Arnold-Chiari and Dandy-Walker malformations.

The morphological findings in 12 cases of Arnold-Chiari and 3 cases of Dandy-Walker malformations are described and compared to those in 2 cases of congenital hydrocephalus of unknown origin, 1 case of isolated meningo-myelocele and 2 cases of normal newborn brains. Lesions common to both types of malformation indicate a same time-related embryogenetic defect of the roofplate of the rhombencephalon, that does not allow the inferior vermis and the choroid plexus to turn inward into the fourth ventricle.