1. In accordance with the views held by Chatton and Alilaire (1908) and by Roubaud (1909), I have seen that a true trypanosome-stage occurs in the life-history ofCrithidia calliphorae. These try-panosomes have an undulating membrane and cannot be considered as herpetomonads with their blepharoplasts situated behind the nucleus and a completely internal flagellum (Woodcock), nor are they herpetomonads with the flagellum bent back along the body of the cell (Mackinnon). This existence of a trypanosome stage in the life-history ofCrithidiais quite comprehensible if we consider that in many cases trueTrypanosomataare changed intoCrithidiaewhen they pass from the blood into the gut of an invertebrate host (T. gambiense, T. lewisi, Schizotrypanum cruzi, etc.).2. There is no sharp difference between the generaCrithidiaandHerpetomonas. InCrithidia calliphoraeforms may be observed without any trace of an undulating membrane (Diagram I, Fig. 1); inHerpetomonas calliphoraesuch an organellum may be sometimes present (Diagram IX, Fig. 4).Still I think that it is not permissible to throw these two genera together, as, generally speaking, an undulating membrane is present inCrithidiaand absent inHerpetomonas.3.Herpetomonasis often biflagellate, even when no signs of cellular division are present and the flagellates multiply less actively than in monoflagellate stocks. Still I agree with Patton that this biflagellate condition is too inconstant to permit of its being recognized as of generic significance.4. The structure of the blepharoplast ofHerpetomonasis distinctly like that of a nucleus, with peripheral chromatin and one or more central granules. It is true that the behaviour of these granules during the division is not so regular as that of the karyosomes of real nuclei, nevertheless, I think that the aspect of the blepharoplast ofH. calliphoraeis a strong argument in favour of Schaudinn's, and also Minchin's, view that this organellum must be regarded as a specialized nucleus (kinetonucleus).5. The flagellum ofCrithidia calliphoraeis constructed in the same way as that ofTrypanosoma lewisi: a basal granule is present and the new flagellum is formed by the production of a new flagellar root by fission of the basal part of the old flagellum. InHerpetomonas calliphoraea basal granule (at the base of the flagellum) and a marginal granule (at the point where the flagellum reaches the surface of the cell and becomes free) are present. The part of the flagellum situated between these two granules is the rhizoplast. The marginal granule first divides; then follows the fission of the rhizoplast and basal granule; lastly, from the marginal granule a new flagellum grows out, so closely entangled with the old one that it often seems as if the production of the new flagellum occurs exclusively by fission.6. The extranuclear chromatoid granules ofH. calliphoraeconsist of volutin. They are numerous during the preflagellate stage and disappear gradually during the flagellate stage; in the post-flagellate stage they are few in number or altogether absent.This behaviour suggests that volutin inH. calliphoraemay act as a sort of nutritive reserve-substance. In degenerating individuals the volutin granules become extremely numerous, an abnormal condition observed also inTrypanosomata(“ volutinosis ”). Consequently volutin seems to play the double part of a nutritive substance, and product of degeneration; this, however, likewise holds for many other nutritive substances (fat, glycogen, etc.).7. The blepharoplast ofH.'calliphoraeshows cyclical changes. During the formation of the post-flagellate stages the achromatic substance is lost and only the chromatic portion remains. The blepharoplast is smallest at this period. The preflagellate stages possess a larger blepharoplast, and this organellum reaches its maximum dimensions during the flagellate stage. I am not able to say whether or not these cyclical changes have any special importance; perhaps they have something to do with the regulation of the normal relation between nucleus and protoplasm.8.H. calliphoraewas observed by me only in the gut of the fly and does not seem to be transmitted hereditarily, as is the case inCrithidia melophagia(Swingle, 1909; Porter, 1910).