222 PATHOGENIC MICROORGANISMS. 



qu-ito. Certain of these crescent-forms give out flagella 

 which are motile filaments containing chromatin, and these 

 loose filaments penetrate and fecundate other crescent-forms. 

 And the fecundated bodies are after this able to penetrate the 

 epithelium of the intestines and travel between the muscular 

 fibres of the intestines. There is therefore a differentiation of 

 sex between the crescent bodies : those becoming flagellated are 

 the male elements, the microgametocytes ; and the others, 

 which do not become flagellated, are macrogametes, the fe- 

 male. These macrogametes after fecundation develop between 

 the muscles of the intestines of the mosquito, becoming sur- 

 rounded with a capsule and acquiring the characteristics of 

 typical sporozoa. After a while its nucleus divides into a 

 number of smaller ones, which in their turn become the 

 nucleus of the dividing cell itself, the sporozoite. These 

 sporozoites are set free by rupture of the capsule of the 

 sporozoa, and are scattered throughout the body of the mos- 

 quito, some finding lodgement in the tubules of the salivary 

 glands, and when the insect again stings man, sporozoites are 

 inoculated into him together with the irritating secretion of 

 the gland. 



This cycle lasts in the mosquito from eight to ten days, and 

 varies with the species of the parasite. It is likely that this 

 represents the whole life of the malarial parasite, and it has 

 been demonstrated that the infected mosquito does not trans- 

 mit the malarial parasite to its larva, the two life-cycles in 

 man and the mosquito being sufficient to explain the known 

 practical facts. 



The three varieties of the malarial parasites found in the 

 human blood differ as to size, distribution of their pigments, 

 in the number of daughter cells produced from one parasite, 

 and the length of time required for the completion of their 

 life-cycle. One form, the tertian (Figs. 70 and 71), requires 

 forty-eight hours ; another, the quartan (Figs. 72 and 73), re- 

 quires seventy-two hours ; and a third form, the sestivo -autumnal 

 (Figs. 74 and 75), has an indefinite life-cycle. Occasionally 

 there are seen what are known as the double -tertian and 

 double-quartan forms of malarial fever, in which the fever is 



