[144] Fantham, _Parasitology_, ii, p. 392. The Spirochætes form an interesting chapter in the evolution of parasites. There are free living forms, parasitic forms in the guts of both vertebrates and invertebrates, and blood-inhabiting forms. These probably represent the order of evolution of parasitism. The blood-inhabiting forms are pathogenic to warm-blooded hosts. We must now consider the blood Spirochætes and the Treponemata (organisms of syphilis and of yaws). THE SPIROCHÆTES OF THE BLOOD. There are at least two important human parasites included hereunder:-- (_a_) _Spirochæta recurrentis_ (=_S. obermeieri_), (_b_) _Spirochæta duttoni_. More is known of the life-cycle of _Spirochæta duttoni_, and it will be convenient to consider that first. *Spirochæta duttoni*, Novy and Knapp, 1906. The specific name _duttoni_ was also given, independently, to this parasite in 1906 by Breinl and Kinghorn. _S. duttoni_ is the pathogenic agent of African tick fever in man, prevalent in the Congo State and other parts of Africa. The full-grown organism is about 16 µ to 24 µ long, and has pointed ends. It is 0·25 µ to 0·5 µ broad. P. H. Ross and Nabarro were among the earliest to see a spirochæte in the blood of patients in Uganda. It is transmitted by the tick, _Ornithodorus moubata_. In the blood of the patient some of the spirochætes may show, after staining, lighter and darker portions (chromatin dots) and evidence of the possession of a very narrow membrane (fig. 54). The mode of division has already been discussed. Periodicity in the direction of division was first described by Fantham and Porter,[145] (1909). Just before the crisis in African tick fever, Breinl has stated that _S. duttoni_ becomes thinner in the spleen and bone-marrow and rolls up into skein-like forms, which are surrounded by a thin “cyst” wall (probably the periplast). Such occur in apyrexial periods. Inside the cyst the spirochæte breaks up into granules. Balfour and Sambon have described somewhat similar rolled up forms, breaking into granules, inside the red blood cells of Sudanese fowls in the case of _S. granulosa_ (possibly only a variety of _S. gallinarum_). The intracorpuscular stage is not definitely established. [145] _Proc. Roy. Soc._, B, lxxxi, p. 500. The granule phase, however, is an essential one in the invertebrate transmitter (fig. 54_c_). In 1905,[146] Dutton and Todd proved experimentally that _O. moubata_ transmitted _S. duttoni_. They fed ticks, obtained from Congo native huts in which infected persons lived, on monkeys and the latter became infected. Dutton and Todd also found the offspring of infected ticks to be capable of transmitting the infection to experimental animals. They concluded that _O. moubata_ was a true intermediate host. [146] _Liverpool Sch. Trop. Med._, _Memoir_ xvii; _Lancet_, Nov. 30, 1907, p. 1523.