[Illustration: FIG. 37.--_Trypanosoma lewisi_, from rat’s blood. A, ordinary form; B, small form; C, D, stages in equal binary fission; E, elongate form (_longocaudense_ type), resulting from division as seen in D; F, unequal binary fission; G, H, multiple fission into four and eight; I, small form; J, binary fission of small form; K, division rosette. × 2,000. (After Minchin and Thomson.)] _Life Cycle in the Vertebrate Host._--After infection of a rat, the trypanosomes usually appear in the animal’s blood in five to seven days. This incubation period applies either to a natural or an artificial infection. The trypanosomes first observed in the rat’s blood are diverse in form (fig. 37), being small, medium and large in size. This diversity is explained by the rapid multiplication taking place. A trypanosome may divide by equal longitudinal fission (fig. 37, C, D), but more commonly multiple fission occurs (fig. 37, G, H), and is unequal. Rosette forms are produced, in which the parent form can be recognized by its long flagellum (fig. 37, H) and attached to it are daughter individuals, smaller in size, from which flagella are growing. Minchin and J. D. Thomson (1912) find that the daughter forms may be set free sometimes with a crithidia-like facies (fig. 37, I), the blepharoplast being anterior but near to the nucleus. The daughter forms, when set free, may themselves divide by binary or multiple fission, in the latter case forming rosettes (fig. 37, K). Rosette forms were described by Moore, Breinl and Hindle in 1908. Lingard, some years ago, described as a distinct species, _T. longocaudense_, certain forms with markedly elongate posterior ends (fig. 37, E). According to Minchin, “these forms appear to arise by binary fission” (fig. 37, D). These long drawn-out forms “are of constant occurrence and very numerous at a certain stage of the multiplication period.” It is about the eighth or tenth day after infection that the multiplication of _T. lewisi_ is at its maximum in the rat’s blood. About the twelfth or thirteenth day the trypanosomes seen in the blood appear uniform. According to Minchin (1912)[91] the rat “gets rid of its infection entirely sooner or later, without having suffered, apparently, any marked inconvenience from it, and is then immune against a fresh infection with this species of trypanosome.” There is, then, a cycle of development in the vertebrate host. Minchin notes that the records of the pathogenicity of _T. lewisi_ in rats, causing their death, need further investigation. [91] “Protozoa,” p. 294. _T. lewisi_ inoculated into dormice (_Myoxus nitela_) and jerboas may become pathogenic thereto.