up these tiny organisms roll up in a ball with a thick coating 

 and, much after the fashion of the seeds of plants, lie dor- 

 mant upon the grass blades, never developing further unless 

 by chance devoured by a sheep. Once back in the digestive 

 system of the sheep, the tiny coated ball or cyst develops into 

 the small worm with which this history started, and makes 

 its home once more in the liver of the sheep. This singular 

 life history presents the spectacle of three generations of 

 varying forms, choosing different hosts, but all parasitic in 

 the adult condition. And the whole purpose of this compli- 

 cated process it must be remembered is simply to provide for 

 getting the descendants of the liver-fluke into the liver of 

 another sheep. 



Many parasites solve their life problem by such an alterna- 

 tion of generations as has been described and of these the 

 one which interests us tonight because of its near relation to 

 ourselves in the genus Plasmodium. 



Plasmodium is one of the smallest of animals, merely a sin- 

 gle cell, and lives as a parasite in the red corpuscles of the 

 blood of higher animals, chiefly in man and birds. The host 

 suffers because the Plasmodium changes the very necessary 

 substance of the red blood corpuscle, viz, the haemo- 

 globin into granules of a supposably poisonous substance 

 called malenin. Every 48 hours (72 hours in another spe- 

 cies) a host of spores and melanin granules is set free in 

 the blood; these spores infect new corpuscles and the process 

 is repeated, over and over again. The melanin granules 

 are supposed to exert a poisonous influence upon the body 

 causing chill, followed by fever and sweating — in brief — ma- 

 laria. 



The derivation of the word {mala, bad-ffr?'rt, air) suggests 

 the theory long held as to the cause of malaria. The night 

 air of low, swampy regions has been held responsible. Drink- 

 ing water, also, has been accused of being the means of in- 

 troducing the malarial parasite into the system. But it 

 remained for the microscopist to discover the real agent of 

 infection. 



21 



