PROTOZOA CAUSING DISEASE 415 



at least, are known. They differ in certain characteristics of 

 morphology and life cycle but closely resemble each other 

 on the whole. These organisms are Plasmodium vivax, the 

 cause of tertian malaria in man, Plasmodium malarice, the 

 cause of quartan malaria, and Plasmodium falciparum, the 

 cause of malignant or tropical malaria. 



The organism is present in the blood stream of individuals 

 infected with the disease. It attaches itself to and probably 

 penetrates the red blood corpuscles and develops in the 

 interior of the cell so the corpuscle is practically entirely 

 filled with the organism, causing it to become somewhat 

 swollen. The organism then segments to form a rosette of 

 bodies which round off to form small spores called 

 merozoites. These become free by the breaking to pieces 

 of the red blood cell and in turn attach themselvs to other 

 cells and begin the cycle again. This may be repeated over 

 and over. It is termed the asexual part or phase of the 

 life cycle. The organism may be taken in with blood from 

 a patient by a mosquito belonging to the genus Anopheles 

 (or to a related genus), whereupon it passes through 

 several distinct stages of development. In the body of the 

 mosquito two types of cells are formed from the spores 

 taken in with the blood, male cells and female cells, called 

 respectively microgametes and macrogametes. Fertilization 

 of the macrogamete by fusion with a microgamete occurs, 

 the resultant cell being termed a fertilized egg or ookinete. 

 This makes its way by boring into the gut wall of the 

 mosquito, where it becomes encysted and enlarges to form a 

 tumorlike swelling. The contents then break up to form a 

 considerable number of spherical bodies known as sporo- 

 blasts, which in turn subdivide and produce within them- 

 selves great numbers of delicate filaments called sporozoites. 

 These are eventually freed by breaking of the cyst and pass 

 out into the body cavity of the mosquito. Some of them 

 eventually make their way to the salivary or poison gland 



