378 GENERAL PRINCIPLES 



but at intervals another type of development occurs side by 

 side with the spores. In this case the enlarged parasitic cell in 

 the blood corpuscle does not divide into a number of spores, but 

 becomes much elongated and cresent-shaped. Now, however, 

 for further development a change of host is necessary and this 

 host must be one of a few species of mosquitos (Anopheles). 

 In the stomach of the mosquito the crescents just described be- 

 come differentiated into two classes. In the one class the 

 crescents become rounded and motionless, while in the other 

 division occurs and a number of very long and slender motile 

 bodies are formed. These are female and male gametes re- 

 spectively, and a fusion takes place between them as in fertili- 

 zation. The zygote now becomes motile. It works its way 

 into the wall of the digestive tract where it remains for about 

 eight days while undergoing further development. This is 

 called the sporocyst stage. During this period it grows to an 

 enormous size, it first divides into a number of cells and these 

 then each develop a vast number of sporozoites, very long and 

 very slender spindle-shaped motile spores. By the bursting 

 of the sporocyst the spores escape into the body cavity and thus 

 gain access to the salivary glands. For some reason they work 

 their way into the salivary glands and their ducts, and hence, 

 when the mosquito next punctures the skin of an uninfected 

 person some of the sporozoites are carried with the saliva into 

 the wound, and the victim is thus inoculated with malarial 

 virus. The sporozoites are merely another type of spore. 

 They attack the red blood corpuscles in the same way as in the 

 stage with which we began. 



769. We see that the malarial parasite completes its life 

 history only by transferring from man to mosquito. Both 

 hosts are necessary. This is probably also true of the try- 

 panosomes, as is indicated by the more recent investigations. 



770. The fever days of malaria are the days in which the 

 new generation of spores are set free by the disintegration of the 



