Alobar holoproscencephaly - features,single midline monoventricle (or holosphere). lateral and third ventricles are absent.absent midline structures. absent septum pellucidum.agenesis or hypoplasia of the corpus callosum absent interhemispheric fissure and falx cerebri thalami fused Differentials,Alobar holoprosencephaly Semilobar holoprosencephaly Hydranencephaly Severe hydrocephalus