THE SIMPLEST ANIMALS 313 



and frees the spores, which fasten themselves to new blood- 

 cells and repeat the development. Each time the spores are 

 freed by breaking of the blood-cells the patient has the chill 

 and fever which are characteristic of the disease. It is well 

 known that malarial attacks commonly occur on alternate 

 days, that is, every forty-eight hours. This means that a 

 common form of malarial parasite requires two days for de- 

 velopment in a blood-cell. Another variety of malarial para- 

 site takes three days for development into spores, and so the 

 chill of the patient comes at intervals of three days. Still 

 another form of malaria, most common in the tropics, and 

 exceedingly difficult to cure, may appear daily; and the 

 parasites take twenty-four hours for their development in 

 blood-cells. 



All these oft-repeated cycles of development in human 

 blood-cells are simply growth and cell-division, typical 

 asexual reproduction. When human blood is sucked into 

 the stomach of a mosquito, pairs of certain spores unite (a 

 true case of fertilization) and the zygote (combined cell) 

 penetrates the stomach-wall of the mosquito and becomes 

 encysted. Sometimes there are several hundred such cysts 

 in the stomach of a single mosquito. Each of these encysted 

 parasites eventually divides into thousands of slender thread- 

 like bodies (sporozoites), which by way of the lymph-tubes 

 get scattered in all parts of the mosquito's body, especially 

 in the poison (salivary) glands. It is these sporozoites which 

 are carried along with the poison when a mosquito plunges 

 its beak into human blood-vessels; and each sporozoite 

 enters a red blood-cell, where it divides into 6 to 24 spores. 



The full development in a mosquito's body need not take 

 over ten days in warm weather. 



When once started in human blood the repeated cycles 

 may go on for a long time, or spores remain which may long 

 afterwards start a new attack of disease. This can be pre- 

 vented by the use of quinine, which kills the parasites, especially 



