Ischemia occuring as early as 26 weeks of gestation results in necrosis which heals without obvious reactive astrocytosis. A smooth-walled porencephallic cavity develops this way. An insult timed in the second trimestr also disturbs the neuronal migration resulting in polymicrogyria. Lesions arising later in the third trimestr and perinatal period heal with gliosis but without abnormal gyral pattern.