Congenital clefts in the cerebral mantle associated with hydrocephalus result from processes essentially agenetic which act upon the fetal nervous system not later than the 2d mo. of gestation. These may be the result of general factors which affect the embryonal cerebral walls primarily through inhibition of growth and differentiation of the medullary tubes. Circulatory disturbances, hydrocephalus, and fortuitous necrotizing processes may never occur or may occur subsequent to the agenesis. The earlier these obscure general factors come into play, the more widespread are the malformations in the entire neuraxis. In the case of the congenital cleft, the term porencephaly merely covers our ignorance as to their causal origin and obscures from view their formal origin which is distinct from that of the common encephaloclastic porencephalies.