1 64 THE PROTOZOA 



the blood. Manson ('96) was one of the first to call attention to the 

 fact that the formation of these so-called degeneration forms, which 

 occurs only at the time when the blood is exposed to the air, is 

 evidence of the beginning of extra-corporeal life. He suggested a 

 theory that the Polymitus forms are flagellated spores, the extra- 

 corporeal homologue of the intra-corporeal spores. He further 

 suggested that, since the parasite is normally incased within a blood- 

 corpuscle, it is unable to leave the host by its own efforts and must 

 be removed by some blood-eating animal, probably a suctorial insect, 

 such as a mosquito, common in swampy, malarial regions. At the 

 same time ('96) Laveran, in France, proposed an identical hypothesis. 

 Subsequent investigation has given the complete confirmation of this 

 hypothesis. The work of Major Ross, in India, of Koch, Grassi, and 

 others, elsewhere, has established beyond a doubt, that extra-corporeal 

 life of the parasite is spent in the mosquito, and that the disease is 

 spread by these insects through inoculation. About eight or ten days 

 after drawing blood from a malaria patient, the insects are able to 

 transmit the germs to new hosts by inoculation through the proboscis. 

 After a number of experiments, Ross and Grassi found that certain 

 genera of mosquitoes, e.g. Culex sp., are incapable of fostering the 

 human parasite, while all species of the genus Anopheles are particu- 

 larly susceptible. 



The history of the parasite in the mosquito has been variously in- 

 terpreted. Manson ('96, '98), on a priori grounds, suggested that the 

 Polymitus form is developed after the blood is taken from the host 

 into the colder digestive tract of the insect, the change of medium 

 acting upon the parasite in the same way that the air does. Here, he 

 argued, it penetrates an epithelial cell and repeats the life-history of 

 an ordinary form, sporulating and increasing by auto-infection. Mac- 

 Callum ('97), however, described a true conjugation between a Poly- 

 mitus form and a free pigmented parasite in a very similar organism 

 (Halteridium Labbe), which is parasitic in the blood of the Ameri- 

 can crow. In this case the impregnated Halteridium slowly changes 

 form, becoming elongated and more or less worm-like, and moves 

 about in the blood-plasm. Thus in this case the Polymitus form 

 corresponds to a spermatozoon, and the ordinary individual, as in 

 Coccidium, to an egg. Ross and Grassi, finally, have demonstrated the 

 same relation in Plasmodium malaria ; the Polymitus form fuses with 

 an ordinary individual in the intestine of the mosquito, and as in 

 Halteridium^ the copula becomes a motile individual which, after a 

 short period, penetrates the epithelial cells lining the digestive tract 

 (Fig. 90, a e). Here it resembles one of the Coccidiida, growing at the 

 expense of the cell-host and finally sporulating. The spores do not 

 form protective coatings, but divide at once into sporozoites (/, g). 



