An exceptional form of cell, but a beautiful manifestation of surface-tension (or so I take it to be), occurs in Trypanosomes, those tiny parasites of the blood that are associated with sleeping-sickness and many other grave or dire maladies. These tiny organisms consist of elongated solitary cells down one side of which runs a very delicate frill, or “undulating membrane,” the free edge of which is seen to be slightly thickened, and the whole of {267} which undergoes rhythmical and beautiful wavy movements. When certain Trypanosomes are artificially cultivated (for instance _T. rotatorium_, from the blood of the frog), phases of growth are witnessed in which the organism has no undulating membrane, but possesses a long cilium or “flagellum,” springing from near the front end, and exceeding the whole body in length[313]. Again, in _T. lewisii_, when it reproduces by “multiple fission,” the products of this division are likewise devoid of an undulating membrane, but are provided with a long free flagellum[314]. It is a plausible assumption to suppose that, as the flagellum waves about, it comes to lie near and parallel to the body of the cell, and that the frill or undulating membrane is formed by the clear, fluid protoplasm of the surface layer springing up in a film to run up and along the flagellum, just as a soap-film would be formed in similar circumstances. [Illustration: Fig. 91. A, _Trichomonas muris_, Hartmann; B, _Trichomastix serpentis_, Dobell; C, _Trichomonas angusta_, Alexeieff. (After Kofoid.)] This mode of formation of the undulating membrane or frill appears to be confirmed by the appearances shewn in Fig. 91. {268} Here we have three little organisms closely allied to the ordinary Trypanosomes, of which one, Trichomastix (_B_), possesses four flagella, and the other two, Trichomonas, apparently three only: the two latter possess the frill, which is lacking in the first[315]. But it is impossible to doubt that when the frill is present (as in _A_ and _C_), its outer edge is constituted by the apparently missing flagellum (_a_), which has become _attached_ to the body of the creature at the point _c_, near its posterior end; and all along its course, the superficial protoplasm has been drawn out into a film, between the flagellum (_a_) and the adjacent surface or edge of the body (_b_). [Illustration: Fig. 92. Herpetomonas assuming the undulatory membrane of a Trypanosome. (After D. L. Mackinnon.)]