study of the cells of the blood, his studies on vital staining and the selective action of methylene blue on the nervous system, the use of methylene blue in the study of the oxidations and reductions occurring in tissues, and his extensive studies in immunity. This experience, covering a period of twenty-five years, led Ehrlich to the belief that "for each specific parasite a specific curative drug must and could be found." And upon this assumption he began his experiments.
To appreciate thoroughly the difficulties of this task and the magnitude of the results, it must be understood that Ehrlich proposed a sterilization of the body in so far as the microorganism, against which the specific remedy was aimed, was concerned. The destruction of bacteria or protozoa outside the body by chemical means is a commonplace of surgical and public health measures; but the destruction of living microorganisms within the living body had never, until Ehrlich accomplished it, been possible without, at the same time, destroying also, in part or in toto, the cells of the host. To avoid the latter it was necessary, therefore, that the protozoa-destroying substance should have a specific chemical affinity for the protozoa in question, but little or no chemical affinity for the cells of the host.
It is impossible to give the details of Ehrlich's seven years of work on this problem; a brief description of the main results must suffice. The first work was done with trypanosomes, the mouse, which could be readily infected, being used as an experimental animal. After testing, with the aid of his assistant, K. Shiga, many hundreds of dye-stuffs, some old and some new, one, a member of the benzidin group, was found which retarded the progress of the trypanosome infection for several days. This led to a limitation of the experimentation to a study of the synthetic products of the benzidin group, many of which were made for the first time by Ehrlich and his assistants. The result was the discovery of a substance which exerted an actual curative effect upon trypanosomiasis. This substance, a red dye destroying trypanosomes, was given the name trypan red (trypan roth). If twenty-four hours after mice had been infected with the trypanosome of Mal de Caderas, a single injection of this dye was made, animals which ordinarily died in four to five days went on to permanent recovery. The blood, twenty-four hours after injection, was found to be free of trypanosomes, which indicated that the effect of the injection was to destroy absolutely every infecting protozoan. Thus was demonstrated for the first time the possibility of completely sterilizing the animal body by a chemical disinfectant without injury to the cells of the host.
In the course of this work an interesting observation was made. If, instead of a dose necessary to destroy all the trypanosomes, a slightly smaller dose was injected, the trypanosomes would disappear from the circulation for a short time and later reappear. If such