injection was repeated at intervals, the period of disappearance of the trypanosome would gradually shorten until finally the drug would have no effect on the infecting organism; in other words, a strain of trypanosomes had been developed which were resistant, immunized as it were, to trypan red and this resistance could be transmitted through many generations. Also, it was found that trypan red was a curative agent only for the infection in mice; on the trypanosome diseases of larger animals, as horses and cattle, it had no curative effect. However, the experience with trypan-red pointed the way to a solution of the difficulty; either a drug must be found which by a single injection would kill every parasite, or several different drugs must be used, which, acting on the same parasite, and thus allowing a combination treatment, would lead to a cure without the danger, to the host, of a single massive dose. It is impossible in the scope of these lectures to follow in detail Ehrlich's work or to go into the complicated chemistry of the substances used. It must suffice to say that as the work went on, Ehrlich and Weinberg found a substitution produced of trypan-red, amidotrypan-red, which destroyed the virulent parasite of nagana, the tse-tse fly disease, and that Mesnil and Nicolle, using the blue and violet azo-dyestuffs, prepared a trypan blue and trypan violet which caused the disappearance of the parasites of nagana, surra and mal de Caderas.
Another line of progress was through various combinations of anilin with arsenic. Before Ehrlich entered this field, Bruce had found arsenic to be a drug of value in treating the trypanosomiasis of horses (surra) and Thomas had found that atoxyl, a combination of arsenic and anilin, would cure a large percentage of infected animals. This latter substance had also been used in the treatment of the human disease, sleeping sickness. Ehrlich made a thorough study of arsenic compounds, and the result was the combination, arsenophenylglycin, a single dose of which absolutely and permanently cures all animals suffering from trypanosome infection.
At about this stage of the development of chemotherapy, Uhlenhuth and Salmon published an account of the brilliant use of atoxyl in the treatment of syphilis, which as we have mentioned, is due to a protozoan, the spirocheta pallida. Unfortunately, as atoxyl sometimes caused blindness, its use was not without danger and therefore not desirable. So Ehrlich immediately turned his attention to the protozoan diseases caused by spirilla, as chicken spirillosis, relapsing fever and syphilis. His labors on these diseases constitute one of the most fascinating of modern laboratory studies and his results are among the greatest of scientific discoveries. His intimate knowledge of the constitution of atoxyl and other arsenic preparations allowed him to proceed rapidly with "a great variety of substitutions, and innumerable arsenic derivatives were synthetized." As human syphilis could be