After the trypanosomes are ingested by the fly during a meal of infected blood, sooner or later multiplication occurs. This development usually begins in the middle or posterior part of the mid gut, and trypanosomes of varying sizes are produced. After the tenth or twelfth day, many long, slender trypanosomes (fig. 30, _a_) are found, which gradually move forwards into the proventriculus. Such long, slender forms represent the limit of development in the lumen of the main gut. The proventricular type, developed about the eighth to the eighteenth or twentieth day, is not infective; it may occur in the crop, but is not to be found permanently there. Between the tenth and the fifteenth days multinucleate forms of trypanosomes are found, and may be styled multiple forms (fig. 30, _b_). Some of these latter may be degenerative. [Illustration: FIG. 30.--_Trypanosoma gambiense_. Development in the fly, _Glossina palpalis_. _a_, slender, proventricular form; _b_, multinucleate form; _c_, _d_, crithidial forms; _e_, infective type of trypanosome found in salivary gland. × 2,500. (After Robertson.)] _Invasion of the Salivary Glands of the Fly._--Long, slender trypanosomes from the proventriculus pass forward into the hypopharynx. They then pass back along the salivary ducts, about sixteen to thirty days after the fly’s feed. The trypanosomes reach the salivary glands as long, slender forms. In the glands they become shorter and broader, attach themselves to the surrounding structures, and assume the crithidial facies (fig. 30, _c_, _d_). As crithidial forms they remain attached to the wall and multiply in the glands. These crithidial stages differentiate into the short, broad trypanosome forms, capable of swimming freely (fig. 30, _e_). Miss Robertson considers the development in the main gut to be indifferent multiplication, and that salivary fluid seems necessary to stimulate trypanosomes to the apparently essential reversion to the crithidial type. The second development in the salivary gland is the essential feature. The short, stumpy forms of trypanosomes (fig. 30, _e_) finally produced in the salivary glands are alone infective. No conjugation of trypanosomes occurs in the fly. Only about 5 per cent. of captive tsetse flies fed on trypanosome-infected blood become infective, but they probably remain infective for the rest of their lives. J. G. Thomson and Sinton (1912)[64] have obtained in cultures the various trypanosome forms of _T. gambiense_ seen in the fly’s main gut. [64] _Annals Trop. Med. and Parasitol._, vi, p. 331. Duke (1912)[65] found _T. gambiense_ in a species of antelope, the situtunga (_Tragelaphus spekei_), on Damba Island in Victoria Nyanza. Wild _G. palpalis_ could be infected therefrom. The antelope may then act as a sleeping sickness reservoir in that district, but men are apparently the chief reservoir. [65] _Proc. Roy. Soc._, B, lxxxv, pp. 156, 483. *Trypanosoma nigeriense*, Macfie, 1913.[66]