We report neuropathological studies of five cases of type II lissencephaly from three fetuses and two infants. This comparative study allowed us to determine the developmental course of the cerebral lesions. Two distinct developmental events seem to generate this type of brain malformation: firstly, an early disturbance in cortex formation, which results both from a disorder of radial migration and a pial barrier disruption; secondly, a late perturbation of cerebral surface organization, resulting in fusion of the cerebral surface. All these features can be related to a primitive meningeal pathology, and more generally, to a neurocristopathy. Accordingly to our observations, this brain malformation appears during both migrational and post-migrational stages and may be considered more like a polymicrogyria than a lissencephaly.