208 PROBLEMS RELATED TO SINGLE-CELLED ANIMALS 



Life cycles in which there are similar primary and intermediate 

 hosts are not uncommon among parasites. The present example 



5i® 



Fig. 108. — Life cycle of the parasitic protozoan, Plasmodium, malariw, 

 that is the cause of quartan malaria. 



1, the parasite, known as the sporozoite, as it is introduced into the blood of man by 

 bite of mosquito; 2, 3, 4 and 5, sporozoite entering red blood corpuscle where the parasite 

 grows and reproduces new individuals by sporulation; 6, destruction of corpuscle resulting 

 in liberation of these new individuals, known as merozoites, and of excretory or melanin 

 granules into blood plasma; 7, 7a, 8 and 8o, formation of male and female gametocytes in 

 red blood corpuscle of man; 9, formation of macrogamete in stomach of mosquito; 9a and 

 9b,f ormationof microgametes in stomach of mosquito; 10 and 11, union of macrogamete with 

 microgamete to form zygote in stomach of mosquito; 12, zygote after penetrating epithelium 

 of mosquito's stomach; 13, sporozoites formed by divisions of zygote within its cyst; 14, 

 female of mosquito that transmits malarial parasite; head of male below; 15, external 

 surface of mosquito's stomach showing swellings produced by encysted stages formed by 

 division of zygotes shown in 13; 16, section of salivary gland of mosquito, showing sporo- 

 zoites which have been freed from cyst and have migrated through the body cavity to the 

 gland cells. They lie ready to be discharged into the blood stream of man when the mos- 

 quito feeds. (After the Leuckart-Chun chart, from Hough and Sedgwick, "Human 

 Mechanism," copyright, 1918, by Ginn & Co., reprinted by permission.) 



