that when equal parts of spirochæte-infected blood, or serum, and normal serum are mixed, the spirochætes survive longer than when the infected blood is mixed with that of an individual who has recovered from relapsing fever. He accordingly concluded that the cause of the crisis in relapsing fever and of subsequent immunity was the development of a germicidal substance in the blood. He was the first to apply serum-therapy in the treatment of relapsing fever. He obtained an anti-spirochæte serum by repeated inoculation of the horse with human spirochæte containing blood. The value of this serum was successfully established by Löwenthal; of 87 patients treated, 43 (49 per cent.) recovered without a relapse.
Novy and Knapp have proved the presence of a powerful germicidal substance in relapsing fever blood by comparing the viability of the spirochætes in blood taken (1) at the onset of the disease, (2) during crisis, (3) twelve to twenty-four hours after disappearance of parasites, and (4) from animals hyperimmunized by repeated infection. In the case of the first, the spirochætes live and are mobile for forty days, and the blood will continue infective up to thirty-seven days; in the case of the second, no living spirochætes are to be found after twenty-four to forty-eight hours; in the third, the parasites die in from thirty to sixty minutes; in the last, they are killed instantaneously.
They further showed, by examining the blood during the decline of the fever in an infected animal treated with immune blood, that the presence of agglutinating and germicidal bodies could be demonstrated in vivo from hour to hour. The parasites are seen to form agglutination rosettes and long agglutination filaments of 70 to 100 μ.
Treated in vitro with hyperimmune serum the spirochætes rapidly become unrecognizable aggregations of granules.
Incubation period.— The incubation period usually lasts from two to ten days. In some instances the attack develops promptly on exposure; it is
