It has been assumed that the arrangement of the arteries at the base of the brain is symmetrical and that the architecture of the circle of Willis provides a structural basis for free anastomotic flow. With increasing utilization of angiography and development of cerebrovascular surgery, there is obvious need for validating this concept. The elaborate embryological evolution of the circle of Willis and the shifting patterns which precede its definitive form predispose to abnormal development which may influence its capacity as a shunt mechanism. Relatively little information in this regard can be obtained from published studies. Investigation of anomalous formation of the cerebral arterial complex has been concerned chiefly with defective development of individual stems and the relation of such abnormalities to embryologic or phylogenetic evolution. Little attention has been paid to the structure of the circle as a whole or to correlation of its configuration with anastomotic potential. To