Insects and Human Health 527 



white cells of the blood, called the phagocytes (page 601). 

 The parasite feeds upon the substance of the cell, growing and 

 multiplying by the simple division of its own substance into a 

 large number of minute spores called merozoites. In its diges- 

 tion the parasite throws off several waste products, among 

 them the black granules which are always present in malarial 

 infection. At last when all the cell's substance is consumed, 

 as at the end of three days in the tertian fever, or of four days 

 in the quartan fever, the wall of the red cell bursts, liberating 

 in the blood of the infected person a horde of merozoites with 

 their waste products. As nearly all cells burst at about the 

 same time, the waste products which are poisonous or toxic 

 cause the attack of fever and chills, or ague, as it is popularly 

 called. 



The number of parasites in the blood is thus increased rapidly 

 with ever greater destruction of the red corpuscles and more 

 severe attacks of fever and chills. Each merozoite repeats the 

 ' cycle of development just described. Most of them reproduce by 

 simple division ; but there are a few which reproduce sexually, 

 not in the human blood, but only in the body of the Anopheles 

 mosquito. This is one of the most important facts discovered 

 by Ross. 



In the mosquito. When an Anopheles sucks the blood of a 

 malarial patient, it ingests with the blood many parasites. All 

 of these except the sexual forms (Fig. 160, a, 6), which are cres- 

 cent in shape, are digested and perish. The sexual parasites 

 continue the development, each form throwing out polar 

 branches that unite with their opposites and produce in this 

 way a fertilized cell. This fertilized body, resembling a sharply 

 pointed rod, pierces and escapes to the outer wall of the mos- 

 quito's stomach (Fig. 160, e, 1,2,3) , attached to which it grows 

 to several times its original size and multiplies by simple division 

 into a number of bodies called sporoblasts. These multiply 

 and develop in a similar way, producing large numbers of minute 

 hair-like organisms, known as sporozoites, which find their 



