PARASITISM 



359 



according to the species of the parasite. When the 

 number of parasites in the blood becomes considerable, 

 a variation presents itself in that some of adult parasites 

 that do not sporulate, but remain large, ovoid or cres- 

 centic bodies, the gametes, or sexually perfect parasites. 

 When the blood is drawn, either for microscopic exami- 

 nation in the fresh state, or by the mosquito, a change 

 takes place in certain of these bodies which become 

 actively amoeboid, show tumultuous cytoplasmic stream- 



FIG. 127. Parasite of tertian malarial fever, a, 6, c, d, e, f, g, Growing 

 pigmented parasite in the red blood corpuscles; h, spores formed by segmentation 

 of the parasite no roset is found, but concentric rings of the cytoplasm divide: 

 i, macrogametocyte; /, microgametocyte with flagella or spermatozoits. 



ing, and then emit delicate filamentous bodies of minute 

 size formerly called " flagella." It was supposed by 

 Manson that these were the essential parasitic forms of 

 the mosquito cycle, but MacCallum showed that they 

 are the spermatozoids of the parasite, and he was able 

 to follow them to the female gametocytes or oocytes and 

 actually observed the process of fertilization in the case 

 of an avian malarial parasite known as Plasmodium 

 (Halteridium) danelewskyi. 

 When fertilization has taken place in the mosquito's 



