In 1925, the cytology of the cerebrospinal fluid, of the meninges and of the perivascular tissues of the brain was studied in cats that had been subjected to subarachnoid injections of trypan blue (Kubie and Shults1). It was found that when the cerebrospinal fluid was drawn from these animals, the proportion of lymphocytes to other cells was consistently higher in the later fractions than in the fluid which drained first. Furthermore, it became clear that after the subarachnoid space was thoroughly drained, the intravenous injection of hypotonic solutions caused the percentage of lymphocytes in the newly formed cerebrospinal fluid to mount even higher. For example, in cat 42 of that study, the lymphocytes of the first drops of fluid constituted 4 per cent of the total number of cells; at the end of the spontaneous drainage, they constituted 80 per cent; after an injection of 0.6 per cent saline