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932 forms are of great variety. After two or three months have elapsed the swarms of trypanosomes in the rat's blood have begun to diminish and eventually disappear. When this has taken place that rat is immune to that particular species of trypanosome. No confirmation of Schaudinn's statement that some part of the development of trypanosomes in the blood is ultra-microscopic has been made, though nitrations of infected blood plasma through a bacterial filter have been experimented with. Fry and Ranken have observed a process of granule-shedding in trypanosomes under the dark-ground illumination (see p. 161).

In some instances trypanosomes appear in the peripheral circulation only at certain periods, as in the case of T. rhodesiense in man; and in others, as in the trypanosome of Athene noctxa, the little owl, they are said to be most numerous in the peripheral circulation during the night-time, and then only during the summer months; in the winter months they are found in the bone marrow. The cycle in the invertebrate host takes place mainly in the digestive tract. Thus, T. lewisi, after being imbibed by the flea, multiplies rapidly, chiefly in the stomach cells, subsequently passing into the hindgut and rectum. Here the parasites assume a leptomonas and crithidia appearance, and occur in large bunches attached by their flagellar ends to the epithelium, where they divide with great rapidity, or they may occur in cyst-like masses within degenerating epithelial cells. Eventually they give rise to the small infective trypanosomes first described by Swellengrebel and Strickland, which occur in vast numbers in the hind-gut and rectum of the flea. The latter becomes infective in four to seven days, and it may remain infective for forty -five days or longer. The flea does not infect the rat through its proboscis, and the parasites are not found in the flea's salivary glands. Theoretically infection can take place in three ways: (a) the flea harbouring the infective forms of the flagellates may be crushed and devoured by the rat; (b) the rat may lick its fur upon which an infected flea has just dejected; (c) the rat may lick and infect with flea dejecta the wound produced by the insect. Of these the second is probably the most common method; the other two are doubtful. Many pathogenic trypanosomes in Africa are transmitted by tse-tse flies (Glossina), and probably these flies remain infective for life. Thus, T. gambiense of man is transmitted by Glossina palpalis, and perhaps by G. morsitans as well; T. brucei (T. pecaudi) (Fig. 233) of big game, and T. rhodesiense of man, by G. morsitans; T. vivax (probably identical with T. cazalboui) by G. morsitans, palpalis, longipalpis, and tachinoides; T. caprœ by G. morsitans; T. dimorphonby G. palpalis; T. grayi, the trypanosome of crocodiles, is also transmitted by this same glossina; T. evansi, of "surra," a disease of horses in India, by Stomoxys nigra, and probably by a tabanus as well.