the blood of malaria patients by Professor Danilewsky, in 1891. This form differed from the ordinary parasites in having many fine, flagelliform appendages, which, breaking away from the parent, would swim about freely in the surrounding fluid (Fig. 3, p). Danilewsky regarded this form as an independent blood parasite, and gave to it the name Polymitus. In a sense, Polymitus has been the key to the life history of the malaria organism, and its history has been the history of the further discoveries upon malaria. In France, Laveran regarded Danilewsky's discovery as indicating some stage in the cycle of Plasmodium malariæ, and not as an independent organism, while Labbé considered Polymitus a degenerate condition of the ordinary parasites and without further significance. The English specialist on tropical diseases. Dr. Manson, found that the malaria parasites, when exposed with the blood to the cooler air, very soon assume the Polymitus form, which he regarded as the extra-corporeal form assumed by the malaria-' organism, for, he argued, the wide distribution of malaria, the spread from individual to individual, can be explained, since the disease is not contagious, only by the assumption of germs outside of the body. Furthermore, he suggested that these germs might be carried from person to person, by insects, such as the mosquito. In the same year, Laveran made an identical suggestion quite independently of Manson. It was not altogether novel, however, with either of these investigators; thus a certain mosquito in central Africa is known to the natives as the fever organism, while the same idea was represented in Theobald Smith's discovery (1893) of the tick as the agent in the transmission of the 'Texas fever' of cattle.
The first positive results on the significance of the Polymitus form were obtained by MacCallum, in Washington, in 1897-'98. A similar Polymitus form is developed by the malaria-organism of birds (Halteridium), and, in the blood of diseased crows, MacCallum observed the filamentous motile bodies of the Polymitus form, break away from the central mass, and unite with an ordinary parasite. The result of the conjugation was a copula with an independent motion, by which it made its way through the surrounding fluids. The later history of the copula was not followed; similar observations were made, by the same observer, upon living malaria-organisms of man, and the Polymitus 'flagella' were seen to unite with larger forms of the parasite.
In the meantime, Major Ross, in India, was working out the mosquito hypothesis of Manson and Laveran, and succeeded in placing that theory upon a very substantial basis. He found black pigment granules, in the intestine and epithelial cells of the mosquito, which were identified as melanin granules of the blood parasite. It was also found, by Ross, that only certain kinds of mosquitoes were selected by the malaria parasites, viz.: various species of the genus Anopheles
