In 1933, I described a method,1 a modification of Dieterle's2 technic, for demonstrating Spirochaeta pallida in single microscopic sections, in which the use of uranium nitrate as a preliminary mordant, originally described by Jahnel,3 prevents the impregnation of nerve fibers in sections of the central nervous system so that they do not obscure the spirochetes in the subsequent impregnation with the silver salts.
More recently several features of the technic have been improved, adding materially to the certainty of result without increasing the time involved. The entire procedure, including the fixation of fresh tissue, can be completed in one and one-half hours.
IMPROVED TECHNIC
Drop fresh blocks of tissue about 5 mm. thick in a 10 per cent solution of formaldehyde heated to 67 C. (152.6 F.) and fix in an oven at 67 C. for ten minutes. (Tissues already fixed with formaldehyde do not