part_of part of part of results in development of diencephalon development epithalamus development The progression of the epithalamus over time from its initial formation until its mature state. The epithalamus is the small dorsomedial area of the thalamus including the habenular nuclei and associated fiber bundles, the pineal body, and the epithelial roof of the third ventricle. GO:0021538 biological_process anatomical structure development epithalamus