THE PROTOZOA AS PARASITES OF MAM 153 



breaks up, setting free the merozoites into the plasma, where 

 each of them proceeds to infect a new corpuscle, into which 

 it bores its way with a pointed end. 



The time which is required to repeat this cycle of asexual 

 reproduction varies with the species of parasite. Thus of 

 the three kinds (at least) of Plasmodium which infest man, 

 P. vivax sets free a generation of merozoites in forty- 

 eight hours, P. malaria in seventy-two hours, and P. 

 falciparum at irregular intervals. The attacks of fever 

 occur when the corpuscles break up, probably because 

 there are then set free substances 

 formed during the metabolism of 

 the parasite which prove poisonous 

 to the host. So it comes about 

 that the fever caused by P. vivax 

 returns every third day, and is 

 known as " tertian ague," and that 

 caused by P. malaria (quartan 

 ague) returns every fourth day, 

 while P. falciparum causes irregular 

 (quotidian) fevers which are more 

 or less continuous. These latter 

 are the " pernicious malaria " of the 

 tropics. For about ten days after 

 infection the parasites are not 

 numerous enough to cause serious 

 trouble. This period is known as 

 the "period of incubation." Many 

 generations of merozoites may 

 succeed one another during the 

 course of the illness, but eventually the resisting powers of 

 the host begin to get the better of the infesting organisms, 

 or, on the other hand, the patient may be about to die. In 

 either case it behoves the parasite to arrange for the continu- 

 ance of its race elsewhere. This is done by the provision 

 of a fresh kind of individual, adapted to transmission by 

 mosquitoes to new human hosts. These, because they give 

 rise in the mosquito to gametes, are known as gamonts, 

 or gametocytes, though the latter name more properly 

 belongs to cells of similar function in the bodies of Metazoa 

 (p. 106). 



Fig. 94. — Gamonts of 

 Plasmodium falciparum. 



a, Before taking on the sausage 

 shape ; i 1 , male gamont in 

 sausage stage ; £ 2 , female 

 gamont in the same stage. 

 The outline is that of the 

 red corpuscle. 



