Formation of cerebral veins can be retraced through eight embryologic stages: before the 4 mm stage, neither arteries nor veins can be distinguished within the primitive network of the head; the 5 mm stage, when three dural plexuses can be identified; the 6 to 12 mm stage, marked by development of anterior dural plexus and lateralization of veins in relation to future cranial pairs; the 12 to 16 mm stage, with development of the anastomosis between anterior and middle dural plexus, and of the pial veins; the 16 to 21 mm stage, marked by regression of primitive sinus of the head and appearance of the anlage of straight sinus and of transverse and sigmoid sinuses; the 21 to 29 mm stage, with appearance of a temporary prosencephalic median vein, possibly implicated in the etiopathogenicity of so-called aneurysms of Galen's ampulla; the 40 mm stage with elaboration of the deep cerebral venous networks; the 60 to 80 mm stage during which the basal veins and Galen's ampulla are formed. After 3 months, the fetal encephalic venous system has attained its adult appearance.