_T. gambiense_ varies from 13 µ to 36 µ in length, its average length being 24·8 µ, as was determined in 1913 by exact biometrical methods by Stephens and Fantham.[58] Three forms of parasite occur. According to Miss Robertson,[59] the relatively short forms from 13 µ to 21 µ long may be regarded as the mature or “adult” type of parasite in the blood. They carry on the cycle in the vertebrate. From them intermediate forms, which are longer than the “adult” but at first have the same breadth, arise by growth. They possess a free flagellum. The intermediate forms grow into long individuals, which are those about to divide. The products of division give rise, directly or indirectly, to the adult forms. [58] _Annals Trop. Med. and Parasitol._, vii, p. 27. [59] _Phil. Trans._, B (1913), cciii, pp. 161–184. [Illustration: FIG. 28.--_Trypanosoma gambiense_. × 1,700. (After Dutton.)] [Illustration: FIG. 29.--_Trypanosoma gambiense_. Development in vertebrate host. _a_, long, slender, _b_, intermediate and _c_, short, stumpy forms, found in the blood; _d_, _e_, _f_, non-flagellate, latent forms from internal organs. × 2,000. (Original. From preparations by Fantham.)] The organism has an elongate body with an anterior or flagellar end and a blunter posterior or non-flagellar end. The protoplasm is finely granular, large inclusions being rare. The central nucleus is oval and large, often containing most of its chromatin concentrated as a karyosome, with small granules only scattered near or on the fine nuclear membrane. The blepharoplast is either rounded or rod-shaped. The undulating membrane is thrown into folds and is bordered by the flagellum. A small basal granule may be present near, or at the actual origin of the flagellum.