Resting stages of some trypanosomes have been found in the internal organs of their vertebrate hosts. The formation of these oval, Leishmania-like bodies will be noted in individual cases later. Similar small oval bodies form an important phase in the life-history of _T. cruzi_, which multiplies normally by multiple fission or schizogony into these oval, daughter elements, and not by binary longitudinal fission in the circulating blood. Polymorphism in trypanosomes (_e.g._, _T. gambiense_, _T. rhodesiense_) is now interpreted as a phenomenon resulting from growth and division.[57] Long, thin forms are those about to divide. Fully mature forms are shorter and broader. Various intermediate types occur and represent growth forms. Formerly, polymorphism was interpreted in terms of sex, thin forms being regarded as males, broad forms as females, while the intermediate types were termed indifferent. Conjugation was not observed, and there is no evidence in support of the sexual interpretation. [57] Robertson (1912), _Proc. Roy. Soc._, B, lxxxv, p. 527. The transmission of trypanosomes from one vertebrate host to another is usually accomplished by the intermediation of some biting arthropod in the case of terrestrial animals, while leeches are usually considered to act as transmitters in the case of the trypanosomes occurring in aquatic animals. Developmental phases of the life-histories of trypanosomes occur in the invertebrate transmitters, and will be considered in individual cases. *Trypanosoma gambiense*, Dutton, 1902. Syn.: _Trypanosoma hominis_, Manson, 1903. _Trypanosoma nepveui_, Sambon, 1903. _Trypanosoma castellanii_, Kruse, 1903. _Trypanosoma ugandense_, Castellani, 1903. _Trypanosoma fordii_, Maxwell Adams. In vertebrate blood _Trypanosoma gambiense_ is polymorphic, for long, thin forms may be seen in contrast with short, stumpy forms, as well as intermediate forms (fig. 29, _a_--_c_). This polymorphism has been interpreted in terms of sex, especially by German investigators, following Schaudinn (see above). However, there is no evidence of conjugation, and the polymorphic forms are more easily interpreted in terms of growth and division, for the long thin forms are potential dividing organisms, and the stumpy or short parasites, with little or no free flagellum, are the adult individuals. _Morphology of T. gambiense in the Circulating Blood._