make physical contact with living cells. In 1878, in Leipzig, a twenty-four-year-old212 medical student, Paul Ehrlich, hunting for a thesis project, proposed using cloth dyes—aniline and its colored derivatives—to stain animal tissues. At best, Ehrlich hoped that the dyes might stain the tissues to make microscopy easier. But to his astonishment, the dyes were far from indiscriminate darkening agents. Aniline derivatives stained only parts of the cell, silhouetting certain structures and leaving others untouched.
