IX] LIFE-CYCLE OF MALAMAL PARASITE 



125 



preventive measures directed against malaria, there has not only 

 been a great increase of practical experience of such action 

 imder particular circumstances, but methods of combating the 

 disease have sometimes become possible which at first sight 

 were not self-evident. As examples may be mentioned the 

 important method of dealing with malaria known as Segregation^, 

 now largely employed in Tropical Africa, and the no less remark- 

 able development of prophylaxis as recently applied among 

 British troops in India, where malaria has been combated not 

 only by direct, but by a host of indirect and often extremely 

 ingenious devices. 



Life-cycle of the Malaria Parasite in Man and in 

 the Mosquito. 



Like many other parasitic Protozoa, the malarial parasite 

 has two periods of multiplication in its hfe-cycle, one in which 

 it multiplies and over-runs its mammalian host, the other in 

 which, to ensure its eventual translation to a fresh host, it 

 multiplies in the body of the mosquito. The first period of 

 multiplication in the blood of man, since it occurs independently 

 of any fusion of male and female elements [syngamy) , is known 

 as the asexual, or, from the fact that multiplication goes on 

 by a process of schizogony , the schizogonous cycle. The period 

 of multiplication, which starts with the entry of blood contain- 

 ing suitable forms of the parasite into the stomach of the 

 mosquito, is dependent on the fertilization of the female element 

 [macrogamete] by a male element {niicrogamete) and is known 

 as the se.xjial or sporogonous cycle, sporogony being the term 

 applied to the analogous part of the hfe-cycle in other Sporozoa. 



The schizogonous cycle can be repeated, as far as is known, 

 an indefinite number of times, and apparently without the inter- 

 vention of any sexual process may keep up a condition of 

 infection of the host lasting for several years. Sporogony, on 

 the other hand, occurs but once, with the formation of a vast 

 number of the forms known as sporozoites, which, after accumu- 

 lating in the sahvary glands of the mosquito, remain without 

 further development until introduced into the blood of man. 



