The introduction of a cytologic method which throws new light on anatomic structure is often the preliminary step toward a further knowledge of pathologic changes. Del Rio-Hortega's description of oligodendroglia1by the use of his silver carbonate method has made possible the following study which shows that these cells undergo a specific alteration which is often the only evidence of a pathologic process in the central nervous system. CLASSIFICATION OF THE INTERSTITIAL CELLS Under the heading of interstitial cells are included neuroglia and microglia cells. The most concise and complete modern description of neuroglia astrocytes is to be found in a publication by Cajal2in 1913. Much of this description was, of course, not new but the gold chloride method which he devised led him to certain definite conclusions. He gave a clearer picture of the fibrous astrocytes of the white matter and the protoplasmic astrocytes of