1-6 Congenital "berry" aneurysm of Circle of Willis
These small (1 to 1.5 cm) saccular dilatations are typically found in the vessels of the base of the brain. They  may be multiple. The pathogenesis is uncertain, but a congenital defect of the media is postulated. These aneurysms are not identified at birth, but seem to develop over years. Rupture results in subarachnoid hemorrhage.