The neuropathological study of corpus callosum agenesis requires a two-phase approach: first it should analyze the putative causal factors, i.e. absence of callosal neurons, commissuration inability or synapse remodelling defect; secondly it has to detect any morphogenetic effects stemming from the absence of commissure such as nonregression of archicortical structures, ventricular enlargement or possible invasion of the remaining telencephaplic commissure by callosal neurons. Absence of callosal neurons due to abnormal corticogenesis gives rise to corpus callosum agenesis without callosal axon, that is without Probst's bundles. Conversely, corpus callosum agenesis occurring secondary to a commissuration default is associated with the presence of callosal axons which travel along the midline instead of crossing, that leads to the formation of Probst's bundles. This inability to cross the midline could be secondary to an obstacle, such as lipoma or as interhemispheric cysts, or primitive due to axonal guidance disturbance. In the latter situation, the commissural defect could affect the other cerebral commissures i.e. anterior or hippocampal commissures, or could become integrated into a more diffuse midline pathology involving both cerebral and extracerebral structures. Finally, it could be assumed that a synapse remodelling defect could lead to atrophy or hypertrophy of the commissure, that occurs in the absence of white matter pathology.