IN A LECTURE delivered in 1927, Spielmeyer1 referred to sclerosis of the cornu ammonis as "this often-described, yet hitherto enigmatic, phenomenon." This statement is still true, even though the pathogenesis of the phenomenon has since been elucidated to a great extent by Spielmeyer himself and his associates. The first gross description of this lesion is generally credited to Bouchet and Cazauvieilh,2 who, in 1825, observed changes of sclerosis or softening in the hippocampus of epileptic as well as of nonepileptic psychopathic patients. The first microscopic examination was done in 1880 by Sommer,3 who found the changes to be restricted to the band of pyramidal cells of the hippocampus. The latter area has since been called "Sommer's sector." In 1889 Chaslin4 described a marginal gliosis in cases of epilepsy and regarded sclerosis of the cornu ammonis as representing merely a site of predilection for such gliosis. Bratz