98 SOME MINUTE ANIMAL PARASITES 



rosette formation whereby they themselves originated. 

 At first the number of parasites in the blood is so 

 small as to cause little inconvenience and to be 

 practically undetected, with the result that for 

 about ten days after infection no fever is felt. But 

 as soon as schizogony on a fairly large scale occurs, 

 then the patient feels ill. This period intervening 

 between the actual bite and the onset of fever is 

 usually referred to as the incubation period. Many 

 generations of merozoites may be produced, but 

 ultimately the health of the patient reacts on the 

 parasite and forces the latter to commence some 

 form of propagation other than schizogony, and to 

 produce forms adapted for life in another host. 

 Thus originate the sexual forms of the parasite 

 (Fig. 21), but unless these are taken up by a mos- 

 quito and an Anopheline they are destined to 

 perish, and the malarial parasites in such cases 

 usually die out. Should a gnat (a Culicine) suck 

 the infected human blood, the malarial parasites are 

 merely digested. 



But what f of the patient? Now, no less than 

 three kinds of malarial parasites are infective to 

 man, and they each require a slightly different 

 period in which to pass through their trophozoite 

 phase and to become schizonts. Plasmodium vivax 

 is the simplest, for it only needs forty-eight hours to 

 accomplish its growing and multiplicative period. 

 As each period of merozoite formation corresponds 

 to an access of fever, the latter is known as " tertian 

 malaria/' and a relapse occurs on the third day. 

 P. malaria, on the other hand, needs seventy-two 



