126 LIFE-CYCLE OF MALARIAL PARASITE [CH. 



Schizogony occurs only in man, and the schizont forms, if 

 ingested by the mosquito, are simply digested. But whilst 

 the greater part of the sporogonous cycle takes place in the 

 mosquito, the early stages, as far as the formation of the sexual 

 forms is concerned, takes place in the blood of man. Thus in 

 the blood of a malarious patient both asexual and sexual forms 

 may be seen ; the former are those concerned in the production 

 of fever and the clinical effects of malaria generally, the latter 

 are of importance only if they are taken up by a suitable 

 mosquito. 



Although three species of Plasmodium occur in man and 

 give rise each to a distinct type of fever, their methods of 

 development and multiplication are essentially the same. 



Asexual cycle. Schizogony. We will commence with a 

 description of the parasite as it is introduced into the blood 

 by the bite of an infected mosquito. At this stage it consists 

 of a small sickle-shaped body, about IO-20//, in length by i-2fj, 

 in diameter. It is known as the sporozoite, and large numbers 

 of these are extruded with the salivary secretion of an infected 

 mosquito, as may be seen by allowing the insect to feed on a 

 drop of glycerine on a slide. The sporozoite consists of a 

 central nucleus surrounded by a uniformly staining elongate 

 mass of cytoplasm. It is pointed at both ends, and by 

 means of flexion is capable of progressive movements in the 

 blood plasma. After being introduced into the blood of a 

 human being three possibilities are open to these sporozoites. 

 They may be killed in the body and thus produce no infection ; 

 they may remain latent in the spleen or some other internal 

 organ until a favourable opportunity to develop presents itself ; 

 and lastly, they may proceed at once to develop and give rise 

 to the characteristic fever after a certain incubation period. 



The sporozoite begins to develop by first boring into a red 

 blood corpuscle and thus becoming an intracellular parasite. 

 Once it has entered the red cell it assumes a rounded amoeboid 

 form, which is known as the trophozoite. 



The young trophozoite is actively amoeboid, giving off 

 pseudopodia and absorbing nourishment from the contents of 

 the red cell. At first it consists of a uniform mass of cytoplasm 



