PHYSIOLOGY OF THE MICROBES 105 



a preparation (Manson). The ideas of specificity " of soil " 

 and of virulence must resolve themselves among the 

 protozoa as among the bacteria into physical and chemical 

 factors. 



The parasitic protozoa of the intestine of one host pass 

 into another host through the external world in the state of 

 spores or cysts. The parasites of the blood cannot enter 

 the blood of a new individual (in nature) except through the 

 intermediation of a blood-sucking insect, and a portion of 

 their life-cycle takes place in this intermediary. Thus the 

 parasite of Laveran is inoculated from man to man by the 

 Anopheles mosquito. In these cases the principal host is the 

 one in which the sexual phase of the life-cycle takes place ; 

 the host in which occurs the non-sexual reproduction is only 

 the secondary or "intermediate host." With regard to the 

 parasite of malaria man is the intermediate or secondary host, 

 and the principal host is the mosquito. The tsetse fly is the 

 principal host of the Trypanosoma gambiense of sleeping 

 sickness. 



Though the protozoa are frequently parasites, they are often 

 themselves attacked by parasites, by bacteria, by chytridiaceae, 

 by saprolegnaceae, and by algae. The algae, however, may be 

 useful commensals, furnishing a food-stuff starch ; but other 

 parasites may kill the protozoa which they infect, provided 

 the latter does not defend itself and overcome its parasite 

 by devouring and digesting it again by intracellular 

 digestion. 



To study the bacteria pure cultures can be made in which 

 they find their food material in solution. The protozoa have, 

 doubtless, methods of nutrition much more complicated, for 

 their culture is more difficult. Several trypanosomes, parasitic 

 in the blood, or, more precisely, in the blood plasma, have, 

 been grown in pure culture by Novy and MacNeal on media 

 to which blood was added. The parasites of malaria (parasites 

 of the red corpuscles) have not been cultivated. The amoebae 

 can be grown on condition that suitable prey is supplied : if 

 in a culture there are only amoebae and as prey a bacterium 



