PARASITISM AND DEGENERATION 353 



a red blood corpuscle by each of these merozoites, the growth 

 of the parasite in the blood corpuscle at the expense of the 

 corpuscle, the maturing and sporulation (or division into new 

 merozoites) of the parasite, and the final breakdown o'f the 

 corpuscle and release into the blood plasma of the tiny active 

 merozoite. Numerous generations of this type are produced 

 in the blood of a patient, but finally a sort of senescence or 

 degeneration of the parasite sets in, and unless there is a fresh 

 infection of the patient from outside, the parasitic host dimin- 

 ishes and finally nearly disappears. If the effects of the para- 

 site have not been too severe during the height of its invasion 

 the patient now recovers. 



For the continued multiplication and persistence of the 

 parasitic species a new process or set of conditions is necessary. 

 If a drop of blood drawn from a malarial patient is examined 

 under the microscope the parasitic individuals abundantly in 

 evidence in this blood will be seen to manifest a curious be- 

 havior within a few minutes. Some of them will move and 

 squirm about with great activity, and extend and retract 

 pseudopodiumlike processes, until finally with great rapidity 

 a few (usually four to six) delicate threadlike flagella or 

 flagellalike processes will shoot out from the body mass and 

 break away from it. These motile flagella are really gametes 

 or sexual cells of one type (the male) while other large nearly 

 immobile sub-spherical parasite individuals which do not be- 

 have as these do are gametes or sexual cells of the other (or 

 female) type. The flagella find and penetrate or fuse with the 

 larger gametes and form a zygote or resting egg cell. 



While the processes just described have been taking place 

 in the blood droplet under our microscope, as a matter of fact 

 this normally takes place in the stomach of a mosquito. For 

 when a mosquito (at least of a certain kind) sucks blood from 

 a malarial patient the blood parasites are of course taken in 

 also and deposited in the stomach where digestion of the blood 

 begins. Now when the zygotes are formed in the mosquito's 

 stomach they do not remain lying in the stomach cavity but 

 move to the wall of the stomach and partially penetrate it. 

 As many as five hundred zygotes have been found in the 

 stomach walls of a single mosquito. The zygote now in- 

 creases rapidly in size, becoming a perceptible nodule on the 

 outer side of the stomach wall, but soon its nucleus and proto- 



