THE PROTOZOA 133 



zoites into the blood-stream where (within an hour) they may again 

 attack and enter other red blood cells. 



The breaking up of the red blood cells takes place at the moment 

 the merozoite is mature, and the chills and fever likewise appear at this 

 time. 



It will be readily understood that when thousands of red corpuscles 

 are thus removed from the circulation, the patient will be pale 

 (anaemic). The chills may be due to the haematozoin granules, which 

 possibly contain poisonous substances. 



This process of the merozoites being thrown out of the red cor- 

 puscles into the blood-stream may continue for some time, but after 

 a while, rounded forms, which throw off tiny filaments, appear. These 

 are the male sexual forms which require a mosquito as host before 

 being able to develop further. The female forms may, however, either 

 go on developing in man or remain dormant, and come forth again at 

 some later date when conditions are favorable. The sexual cycle, al- 

 ready described in the mosquito's body, now begins if the infected 

 individual is bitten by a female Anopheles mosquito. 



The chills always appear at regular intervals, because the incuba- 

 tion period of each of the three kinds of malarial parasites (although 

 differing for each species) is always the same for the same species. 

 Thus the tertian fever species (Plasmodium vivax) "hatches" every 

 third day hence its name ; the quartan (Plasmodium malariae) every 

 fourth day, while the aestivo-autumnal type (Plasmodium immaculatum 

 Laverania), at irregular intervals. In fact, the physician uses this 

 definite incubation period as his clue in diagnosing the case, to find 

 what particular type of malarial parasite has infected his patient. 



After the spores or merozoites are thrown into the blood-stream, 

 many are devoured by the white corpuscles (leukocytes), but those not 

 devoured, again enter new red corpuscles and so continue reinfecting 

 the same patient, although they are unable to infect another. 



The method of communication from one person to another can only 

 come about in the following manner : 



A female mosquito of the genus Anopheles must suck the blood 

 of an infected person if the disease is to be communicated. As soon 

 as the infected blood reaches the changed environment of the mos- 

 quito's stomach, the series of changes begin in the merozoites, which 

 have been described above. It will thus be seen that Plasmodium ma- 

 lariae must not only pass through two stages of a life-cycle, sexual and 

 asexual, but these two stages are unable to develop in a single host, the 

 asexual stage developing in man, and the sexual in mosquitos. 



At this point the question will occur, "How do we know all this?" 

 It is the answer to this question which will give the student (1) the 

 finest illustration of what modern -laboratory methods mean; (2) it will 

 acquaint him with the exhaustive investigations which students of sci- 



