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 huge number of slender, slightly sickle-shaped germs or sporozoites (“blasts,” “zoids,” &c.) are formed. Each oocyst may contain from hundreds to thousands of sporozoites.

When the sporogony (which lasts about 10 days) is completed, the oocyst ruptures and the sporozoites are set free into the body-cavity, leaving behind a large quantity of residual cytoplasm, including pigment grains, &c. The sporozoites are carried about by the blood-stream; ultimately, however, apparently by virtue of some chemotactic attraction, they practically all collect in the salivary glands, filling the secretory cells and also invading the ducts. When the mosquito next bites a man, numbers of them are injected, together with the minute drop of saliva, into his blood, where they begin a fresh endogenous cycle.

There is only one other point with regard to the life-history that need be mentioned. With the lapse of time all trophic and schizogonic (asexual) phases of the parasite in the blood die off. But it has long been known that malarial patients, apparently quite cured, may suddenly exhibit all the symptoms again, without having incurred a fresh infection. Schaudinn has investigated the cause of this recurrence, and finds that it is due to the power of the megagametocytes, which are very resistant and long-lived, to undergo a kind of parthenogenesis under favourable conditions and give rise to the ordinary asexual schizonts, which in turn can repopulate the host with all the other phases. Microgametocytes, on the other hand, die off in time if they cannot pass into a mosquito.

Various types of form are to be met with among the Haemosporidia. In one, characteristic of most (though not of absolutely all) parasites of warm-blooded Vertebrates, the trophozoites are of irregular amoeboid shape; hence this section is generally known as the Haemamoebidae. In another

type, characteristic of the parasites of cold-blooded Vertebrates, the body possesses a definite, vermiform, i.e. gregariniform shape, which is retained during the intracorpuscular as well as during the free condition; this section comprises the Haemogregarinidae. Allied to this latter type of form are the trophozoites of Piroplasma, which are normally pear-shaped; they differ, however, in being very minute, and, moreover, exhibit considerable polymorphism, rod-like (so-called bacillary) and ring-forms being of common occurrence. It is important to note that in a certain species of Haemogregarina (fig. 3) the young trophozoites markedly resemble Piroplasma in their pyriform appearance; and a further point of agreement between the two forms is mentioned below. Lastly there is the Avian genus Halteridium, the trophozoites of which are characteristically bean-shaped or reniform. True Haemogregarines also differ in other slight points from “Haemamoebae.” Thus the young endoglobular trophozoite does not exhibit a ring (vacuolar) phase; and the cytoplasm never contains, at any period, the characteristic melanin pigment above noted. In some species of Haemogregarina the parasite, while intracorpuscular, becomes surrounded by a delicate membrane, the cytocyst; on entering upon an active, “free” period, the cytocyst is ruptured and left behind with the remains of the corpuscle. A very interesting cytological feature is the occurrence, in one or two Haemosporidia, of nuclear dimorphism, i.e. of a larger and smaller chromatic body, probably comparable to the trophic and kinetic nuclei of a Trypanosome, or of the “Leishman-Donovan”

bodies. Schaudinn was the first to notice this character, in Piroplasma canis, and his observation has since been confirmed by Lühe. Moreover, Brumpt has also noticed nuclear dimorphism in the ookinete of a species of Haemogregarina in a leech (as the Invertebrate host)—a highly important observation.

As regards the life-history, the endogenous (schizogonous) cycle is known in many cases. Sometimes schizogony takes the primitive form of simple binary (probably) longitudinal fission; this is the case in Piroplasma (fig. 4) and also in Haemogregarina bigemina just referred to. From this result the pairs of individuals (“twins”) so often found in the corpuscles. In addition, however, at any rate in Piroplasma, it is probable that multiple division (more allied to ordinary schizogony) also takes place; such is the case, according to Laveran, in P. equi, and the occurrence at times of four parasites in a corpuscle, arranged in a cruciform manner, is most likely to be thus explained. Labbé has described schizogony in Halteridium danilewskyi as taking place in a rather peculiar manner; the parasite becomes much drawn-out and halter-like, and the actual division is restricted to its two ends, two clumps of merozoites being formed, at first connected by a narrow strand of unused cytoplasm, which subsequently disappears. Some doubt, however, attaches to this account, as no one else appears to have seen the process. For the rest, schizogony takes place more or less in the customary way, allowing for variations in the mode of arrangement of the merozoites. It remains to be noted that in Karyolysus lacertarum, according to Labbé, two kinds of schizont are developed, which give rise, respectively, to micromerozoites and megamerozoites, in either case enclosed in a delicate cytocyst. This probably corresponds to an early sexual differentiation (such as is found among certain (q.v.), the micromerozoites producing eventually micro-gametocytes, the others megagametocytes.

It has now been recognized for some time that the sexual (exogenous) part of the life-cycle of all the Haemamoebidae takes place in an Invertebrate (Insectan) host, and is fundamentally similar to that above described in those cases where it has been followed. In contradistinction to the malarial parasites, this host, in the Avian forms (Haemoproteus and Halteridium) is a species of Culex and not of Anopheles; in other words, gamete-formation, conjugation and subsequent sporozoite-formation in these cases will only go on in the former. On the other hand, in the case of the Haemogregarines, it was thought until quite lately that the entire life-history, including conjugation and sporogony, went on in the Vertebrate host; and only in 1902 Hintze described what purported to be the complete life-history of Lankesterella (Drepanidium) ranarum undergone in the frog. This view was rendered obsolete by the work of Siegel and Schaudinn, who demonstrated the occurrence of an alternation of hosts and of generations in the case of Haemogregarina stepanovi, parasitic in a tortoise, and in Karyolysus lacertarum; the Invertebrate hosts, in which, in both cases, the sexual process is undergone, being respectively a leech (Placobdella) and a tick (Ixodes). With this discovery the main distinction (as supposed) between the Haemosporidia of warm and of cold-blooded Vertebrates vanished. It was further acknowledged by Schaudinn (under whom Hintze