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236 notwithstanding repeated attempts, failed to communicate the disease by means of these insects. But as we have now reason to believe that there are several strains of pathogenic spirochætes producing disease in man, it may be that these observers have not worked either with the appropriate kind of spirochæte, or with the appropriate species of Acanthia. Thus may we explain the positive results claimed by Sikiel, in Odessa, and by other Russian experimenters. There can be no doubt from the experiments of Philip Ross and Milne in Uganda, of Dutton and Todd on the Congo, and of many others, that the African species, S. duttoni, is normally conveyed by the tick Ornithodoros moubata, and that it can be transmitted not only by the animal that has bitten the infected individual, but by its progeny. The spirochæte has been demonstrated in the egg of the tick by Koch, and Leishman describes certain groups of " coccoid bodies," which may have been derived from spirochætes, in the same situation. It may be in this way that the hereditary transmission of the spirochæte in ticks and lice is effected. According to Leishman the spirochætes are commonly found in ticks' eggs laid in the tropics, but not in those laid in this country; but Balfour believes that these eggs contain his infective granule (see p. 231). Symptoms of a successful infection by the tick appear in from five to seven days.

It would seem that the various insect transmitters act merely in a more or less mechanical way and that there is no essential biological relationship between the parasite and the transmitter, as is the case with the malaria parasite and the mosquito, or with the trypanosome and the tse-tse fly. The spirochæte is abstracted with the blood when the insect transmitter feeds; it multiplies by breaking up into refractile granules* in the gut and Malpighian tubes of the latter as in a culture-tube, and is afterwards passed out with the fæces and deposited on the skin of any