The origin of the oligodendroglia is still a debated question. This cell, which Cajal called the "third element" of the brain, apparently has an origin different from that of the microglia. Del Rio Hortega is an advocate of the mesodermic origin of the microglia, although his view is not yet entirely accepted. With the new conception of the ectodermic origin of the meninges, the question of the embryologic derivation of the microglia again comes to the fore and will certainly be the subject of new investigations. I may anticipate my observations by the statement that in my investigations of the meningiomas I have found numerous microglia cells in the tumors themselves. This fact might be considered as in support of the mesodermic origin of the microglia, as well as an argument against it if other elements of ectodermic origin are to be found in the same tissue. In regard to