The study of arterial and venous anatomy of hemispheric cerebral arteriovenous malformations (AVMs) shows that, beside occlusion, rerouting of the flow, and collateral supply and drainage, the arterial and venous patterns are normal. This can be explained by the fact that the cerebral hemispheres develop late relative to other neural structures, and their fetal vessels become in fact the permanent vessels. This normality indicates that the AVMs develop after the vascular pattern has been determined. Vein of Galen aneurysms, in contrast, demonstrate occasionally an abnormal arterial pattern over the hemisphere, and retention of an embryonic vein, the median prosencephalic vein. This indicates that the fistula occurred at a much earlier stage, actually when the choroid plexuses first developed, about the end of the second month. The occurrence of choroid AVMs similar to the vein of Galen aneurysms, but drained by a normal internal cerebral vein, indicates that the time when the choroid drainage is taken over the internal cerebral vein, represents another critical period.