Two nuclei are present. The main or principal nucleus, sometimes termed the trophic nucleus, is often situated towards the centre of the body; it is frequently of the vesicular type, containing a karyosome. The blepharoplast or kinetic nucleus is posterior to the nucleus, and usually is rod-like. The flagellum arises close to the blepharoplast, and forms an edge to the undulating membrane. It may or may not extend beyond the limits of the undulating membrane. If it does so, the unattached part is known as the free flagellum. Sometimes a small granule is found at the origin of the flagellum. This is the basal granule, and is considered by some to function as the centriole of the kinetic nucleus. The undulating membrane is a lateral extension of the ectoplasm or periplast, and is the main agent in locomotion. It is edged by the flagellum, which forms a deeply stainable border to it. Within the membrane substance, often arranged parallel with its edge, are a number of fine contractile elements, the myonemes. These contractile elements may also occur on the body of the trypanosome. They are easily seen in some large trypanosomes, but are difficult of demonstration in others, owing to their great fineness. [Illustration: FIG. 27.--_Trypanosoma lewisi_. Multiplication rosettes. × 1,000. (After Laveran and Mesnil.)] Multiplication of trypanosomes in the blood is brought about by binary longitudinal fission (fig. 26). Division is initiated by that of the blepharoplast and nucleus. The division may be equal or subequal, whereby differences in size of individuals partly arise. Multiple division by repeated binary fission, without complete separation of the daughter forms, is known in some trypanosomes (_e.g._, _T. lewisi_), and rosettes of parasites thereby are produced (fig. 27). The classification of trypanosomes is very difficult. Laveran (1911)[54] has suggested the examination of the relative length of the flagellum as a diagnostic character, and so arranged these flagellates in mammals in three groups. The first group included those trypanosomes always having part of the flagellum free (_e.g._, _T. evansi_, _T. vivax_); the second group comprised forms without a part of the flagellum free (_e.g._, _T. congolense_), while the third group included forms some members of which have free flagella, while others have not (_e.g._, _T. gambiense_). Bruce[55] (1914) and Yorke and Blacklock[56] (1914) have also devised classifications. [54] _Ann. Inst. Pasteur_, xxv, p. 497. [55] _Trans. Soc. Trop. Med. & Hyg._, viii, p. 1. [56] _Annals Trop. Med. and Parasitol._, viii, p. 1.