High Quality Content by WIKIPEDIA articles! Agenesis of the corpus callosum (ACC) is a rare birth defect (congenital disorder) in which there is a complete or partial absence of the corpus callosum. Agenesis of the corpus callosum occurs when the corpus callosum, the band of white matter connecting the two hemispheres in the brain, fails to develop normally, typically during pregnancy. The development of the fibers which would otherwise form the corpus callosum become longitudinally oriented within each hemisphere and form structures called Probst bundles.