THE PROTOZOA THE DAWN OF LIFE 17 



corpuscle. The nucleus of the parasite now divides repeatedly, 

 after which the parasite divides into a number of spores and 

 bursts the corpuscle, thus liberating the newly-formed spores and 

 the black refuse-matter, which acts as a poison, producing chill, 

 nausea, shivering, and fever in the patient. The liberated spores 

 at once attach themselves, each to a fresh red corpuscle, and a 

 repetition of the life-cycle ensues. 



After the disease has persisted for some time, certain of the full- 

 grown parasites, instead of dividing up, pass, as it were, to a period 

 of rest as either round or blunt crescent-shaped cells (termed 

 " half-moons "). These are sexual cells, which only develop further 

 when the blood containing them is withdrawn from the human 

 host. This sexual cycle of the parasite was discovered by 

 Major Ronald Ross to take place in the gut of the spot-winged 

 mosquito (Anopheles), which he has also proved to be the trans- 

 mitting agent of the parasite of malaria from man to man. 



When the Anopheline mosquito pierces the skin and sucks up 

 the blood of a person suffering from malaria, it also sucks up a 

 number of the resting sexual cells of the parasite, which at once 

 begin to undergo a series of changes ; they become spherical, and 

 the males produce flagella-like sperms, which break away and fuse 

 with and fertilise the female spheres or egg-cells. The fertilised 

 cell takes on an active worm-like form, and partially pushing 

 itself through the wall of the mosquito's gut, feeds on the insect's 

 blood. It soon swells up, divides internally again and again, 

 and becomes enclosed in a firm, transparent cell or cyst. Finally, 

 the cyst-wall is ruptured, and the needle-shaped spores escape 

 and accumulate in the salivary glands of the mosquito, passing 

 out of the mouth of the insect when it stabs a fresh human victim, 

 who thus becomes infected. It is only in species of Anopheles 

 that the parasite can undergo its sexual development and the 

 process of the production of needle-shaped spores, for if the 

 sexual cells are swallowed by an ordinary gnat or mosquito of 

 the genus Culex they are digested by the insect and destroyed. 



These joint discoveries of the malaria parasite by Laveran, 

 and of its sexual cycle in the gut of the transmitting agent, 

 the spot-winged mosquito, by Major Ronald Ross, are of the 

 greatest economic importance and far-reaching in effect. They 

 rendered the cutting of the Panama Canal possible, and have 

 c 



