276 ESSENTIALS OF ZOOLOGY 



of certain of these forms. Life cycles in which there are primary 

 and intermediate hosts are quite common among parasites. This 

 example will illustrate also the relationship of insects to disease- 

 producing organisms. There are three species of human malaria- 

 causing organisms: (a) Plasmodium vivax, which causes tertian 

 fever, is characterized by an attack each forty-eight hours, (b) Plas- 

 modium malariae, which causes quartan fever, is characterized by an 

 attack every seventy- tw^o hours, (c) Plasmodium falciparum, caus- 

 ing estivo-autumnal or subtertian fever, the attacks of which recur 

 each day, or there may be a somewhat constant fever. The parasite 

 (see Fig. 176) may live in the blood of man by a series of asexual 

 generations which may continue throughout the life of the person. 

 The parasite, while in the spore stage, invades the red corpuscles, 

 where it reproduces by a sort of multiple division called sporulation, 

 in which there are numerous nuclear divisions before the mass of 

 cytoplasm divides. The new individuals (merozoites) are freed by 

 destruction of the corpuscle and almost immediately enter new cor- 

 puscles where repetition of events occurs. Some of these merozoites 

 become sexual cells (gametocytes). Part of the gametocytes develop 

 into macrogametes, spoken of as female, and others become micro- 

 gametes which develop from the male gametocyte. If a female 

 Anopheles mosquito bites and sucks blood from this person, the mos- 

 quito becomes infected with gametocytes of the Plasmodium. A union 

 of the flagellate microgametes with the egglike macrogametes takes 

 place in the stomach of the mosquito. The union is commonly called 

 fertilization, and a fused cell or zygote thus formed soon becomes a 

 motile, wormlike form, known as an ookinete. This ookinete enters 

 the wall of the mosquito's stomach where it encysts in the form of a 

 ball with a shell, and is now called an oocyst which grows at the ex- 

 pense of the adjacent tissue. This cyst protrudes like a little wart 

 on the outside of the wall of the stomach. Inside of the oocyst the 

 nucleus divides repeatedly, forming sporohlasts. These enlarge and 

 coalesce, while slender, spindle-shaped sporozoites develop within, 

 each with a chromatin dot as a nucleus. The capsule of this oocyst 

 is crowded full of these sporozoites which may number 10,000 or 

 more, and there may be 500 capsules in one mosquito. Depending 

 somewhat on the temperature, it requires twelve days or more for 

 this development to go on in the mosquito. These little parasitic 

 sporozoites make their way to the salivary gland of the mosquito 



