1) Destructive processes are responsible for most cases of cerebellar microgyria of the trabecular pattern. Erosion and subsequent fusion of the folia produce the disorganized pattern in which the various cellular elements retain their noraml relationship and are capable of normal maturation. Intrauterine infection is responsible for most cases; the evidence is conclusive in some cases, presumptive in others. 2) Faulty genetic coding, as illustrated by the trisomies, may lead to formation of heterotopias. The primitive cells aggregating around the dentate nucleus should be interpreted as matrix cells and not as cells of the external granular layer. Cortical heterotopias with attempted internal organisation also occur; their origin is obscure. The unusual, possibly unique, transposition of the internal granular and Purkinje cell layers observed in one case may be ascribed to faulty formation of the Bergmann glia by analogy with the weaver mouse. 3) It is impossible at present to disentangle the role of genetic and environmental factors in the pathogenesis of the hysraphic malformations. It is possible, however, that defective fusion of the intraventricular cerebellar primordium plays a part in the development of the Dandy-Walker malformation, of midine cerebellar clefts in some cases of occipital encephalocele, and of extra-axial ependymal cysts of the posterior fossa.