The ontogenesis of arachnoid granulations has been analysed in specimens derived from 30 humans, 5 cats and 10 ships. We distinguish the following developmental stages : (i) The arachnoid membrane forms small diverticula that grow toward the superior sagittal sinus. In the majority of cases, this process is followed by an infiltration of mesothelial and connective tissue cells from these diverticula into the dura mater, where it forms the wall of the sinus. (ii) These arachnoid cells form an evagination, giving rise to an arachnoid granulation of simple pediculated form, which, by generating evaginations of second and third order, can become an arachnoid granulation of the so-called complex pediculated form. The wall of the superior sagittal sinus participates in the development of the granulation by the formation of a fibrous capsule. This capsule has openings where arachnoid mesothelium and sinus endothelium are in direct contact.