along these lines it is easy to work out their complete life-histories, and, above all, to find out how they pass from one host to another. In the case of H. muscae domesticae in Madras the flagellate and postflagellate stages are readily found, but owing to the fact that a large number of flies have to be examined before the preflagellate stages can be studied, this important portion of the life-cycle of the parasite is very apt to be missed.
The preflagellate (Plate I., figs. 1 to 3a) stage is usually found in the midgut, the parasites lying in masses within the peritricheal membrane; they are round or slightly oval bodies, measuring on an average about 5.5 μ in breadth, and contain a nucleus, a blepharoplast, and a number of granules. They multiply by simple longitudinal fission or by multiple segmentation (figs. 2, 2a, 3, 3a), so that very soon a large number of these bodies is formed. The flagellate stage is characterized by the formation of the flagellum, a single stout filament which projects freely from a point in close proximity to the blepharoplast (fig. 4); the resulting flagellates elongate and later divide by simple longitudinal division (figs. 5, 6 and 7). The nucleus of the adult flagellate lies about the centre, and the blepharoplast, a large rod-shaped body, is nearly always situated a short distance from the anterior end; the flagellum is a single filament which projects freely almost at once from the rounded anterior end. The early stage of the process of longitudinal fission begins by a splitting of the root of the flagellum, and this, as well as further stages of the same change, led Prowazek to believe H. muscae doinesticae has a double flagellum. Had he, however, studied the method of formation of the flagellum he would not have fallen into this error. It is quite a common thing to find nearly all the adult flagellates in a fly exhibiting
