Though the pathologic diagnosis of intracranial tumors may often be made with considerable accuracy at the time of operation from their gross appearance alone, the subsequent histologic study of the tissues not infrequently controverts the provisional diagnosis thus made, for the cellular architecture of many of these lesions often reveals peculiarities previously unsuspected. However, a proper histologic examination must comprise much more than the routine hematoxylin and eosin stains which may serve merely to add confusion to a classification supported by the gross appearance of the lesion. One of us (Cushing) has pointed out that this is true, for example, of the common tumors ( neurinomas) of the lateral recess, the great majority of which arise from the nervus acusticus. Consequently, if we are to rely on a histologic diagnosis for tumor classification, it is necessary to lean on something more dependable than the mere architectural arrangements of the cells, and