this appearance of a double flagellum. If the mid-guts of a number of flies are next examined in the fresh condition, in some, the flagellates will be seen collecting together towards the rectum, they attach themselves in rows by their flagellar ends to the gut-wall, and now further changes are seen to occur. The most external parasites begin to shorten, and while this change is taking place the flagellum degenerates and is eventually shed (figs. 8 and 9). A palisade of parasites is now formed, the most internal showing the changes noted above, while the most external are seen as round bodies devoid of flagella, and many of these latter forms are seen dividing again (fig. 10). Eventually, all the bodies round up and are connected together by a sticky substance, and these masses of cells constitute the cysts (fig. 11); this I regard as the postflagellate stage. These cysts are passed out in the faeces of the fly en masse and are well adapted to resist desiccation. It is these bodies which are again sucked up accidentally by the flies as they feed. Such is a short account of the life-cycle of H. muscae domesticae; it is exceedingly simple, for I can find no evidence of the complicated sexual changes described by Prowazek.
Turning now to another herpetomonad, H. lygaei, which I have recently described, you will see that it has a very similar life-cycle, only in this case the parasite is almost identical with that of kala-azar. In its preflagellate stage it is a small round or pear-shaped body (Plate II., fig. 1), measuring from 3.5 μ to 4 μ in length; it has a nucleus and blepharoplast and is found lying free in the mid-gut of Lygaeus militaris. The parasite divides, by simple fission into two, and each half again divides, so that four oval bodies are formed (figs. 2, 3 and 4). These bodies now pass on to flagellation somewhat
