In stage 2 of ventral induction, there is formation of the brain and face by the formation of vesticles. Three main vessicles form. During this time, the face also develops such that facial anomalies may accompany brain anomalies during ventral induction. These anomalies include the holoprosencephalies, corpus callosal agenesis, and Dandy Walker. These two processes occur about the same time between two to five months of gestation.
Neuronal migration proceeds from the germinal matrix to the cortex.
Disorders of migration include the heterotopias, agyria-Pachygyria, polymicrogaria. Disorders of histiogenesis include the phakomatosis. In addition, vascular malformations and teratomas may also form at this time. This is the formation of white matter which generally proceeds from more primitive areas to more advanced areas of the brain. Thus one might see this occurring from an inferior to a superior direction and from a posterior to anterior direction.