_, iHF PROTOZOA AS PARASITES OF MAN 



rouiui them so as to form a rosette of little, uninucleate individuals 

 — thf morozoites or schizozoites— which surround some ' residual 

 protophisni ' containing the pigment granules. Next the sheU of 

 the red corpuscle breaks up, setting free the merozoites mto 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 four 

 species of Plasmodium which infest man, P. vivax and P. ovale set 

 free a generation of merozoites in forty-eight hours, P. malaricB 



in seventy-two hours, and P. falciparum 

 in thirty-six to forty-eight hours or at 

 irregular intervals. The attacks of fever 

 occur when the corpuscles break up, 

 probably because there are then set free 

 substances formed during the metaboHsm 

 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 ' benign tertian ague ', 

 and that caused by P. malarice (quartan 

 ague) returns every fourth day, while P. 

 falciparum causes malignant tertian ague, 

 or irregular (quotidian) fevers which are 

 more or less continuous. These latter are 

 the ' pernicious malaria ' of the tropics. 

 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 con- 

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

 of a fresh kind of individual, adapted to transmission by gnats to 

 new human hosts. These stages, because they give rise in the insect 

 to gametes, are known as gamonts, or gametocytes (Fig. 43), 

 though the latter name more properly belongs to cells of similar 

 function in the bodies of Metazoa (p. 79). In the parasites of 

 tertian and quartan fevers they are rounded, in that of pernicious 

 malaria crescent-shaped. They are larger than the schizonts and 

 have more of the dark pigment. 



It is said that the gamonts have no ring-stage in their develop- 



FiG. 43.— Gamonts of 

 Plasmodium falciparum. 



a. Before taking on the sausage 

 shape; 6', male gamont in 

 &ausai;e stage ; b^, female 

 gamont in the same stage. 

 The outline is that of the 

 red corpuscle. 



