Holoprosencephaly; coronal section: A coronal section of the brain in a case of holoprosencephaly reveals a monoventricular cavity, often with incomplete development of the third ventricle, and abnormal development of neocortex. The superior cerebral convolutions tend to follow a transverse pattern, and the interhemispheric fissure may be partially formed or completely absent. The olfactory structures are also usually malformed or absent (hence the term "arrhinencephaly").