THE PROTOZOA 



141 



dealing with malaria man is termed the principal host, since he 

 suffers a disease, while the mosquito is the secondary host, for the 

 parasite is quite innocuous to it, but this usage of the terms is not 

 strictly biological. 



The parasite, when it is introduced into the blood stream of a man 

 by the stab of an infected female mosquito, has the form of an 

 extremely minute spindle-shaped sporozoite. The sporozoite im- 

 mediately attaches itself to a red blood corpuscle, into which it 

 bores its way, and when inside enters into the trophozoite, or feeding 



FIG. 45. A diagram illustrating the stages of the life cycle of Plas- 

 modium vivax found in human blood. 'From Bourne. 



I, -IX. show the schizogonous cycle. In 7. a sporozoite is boring its way into a red corpuscle ; 

 //., young amoeboid phase; ///., a vacuole has appeared near the nucleus giving the characteristic 

 ring form ; IV. and V., pigment (melanin) granules are deposited in the cytoplasm, the parasite 

 has increased in size and exhibits active pseudopodial movements ; VI., nucleus with an equatorial 

 ring of chromatin granules ; VII. and VIII., successive stages of nuclear division ; IX., segmenta- 

 tion of the cytoplasm round the nuclei to form the merozoites which are shown at mer escaping 

 into the blood plasma, spz., sporozoites ; p., pigment granules ; gam., a young game tocyte ; tf , 

 a male gametocyte (microgametocyte) and 2 a female gametocyte (macrogametocyte) of P. xivax, 

 In this species and in P. malaria the gametocytes are not crescent-shaped as they are in P. im- 

 maculatum. (Somewhat diagrammatic after Scbaudinn.) 



stage, assuming an amoeboid form. At first very tiny, it gradually 

 grows until it almost fills the corpuscle ; it acquires a large vacuole, 

 and granules of a dark pigment termed melanin are formed in it. 

 When fully grown the amoeboid movements slow down, the vacuole 

 disappears, and the animal rounds itself off preparatory to entering 

 into a reproductive phase. The form of multiplication undergone 

 is termed Schizogony, and the individual about to pass through it is 

 consequently known as the Schizont. The nucleus divides indirectly 

 a number of times, giving rise to from twelve to sixteen daughter 

 nuclei. As in all multiple fission the cytoplasm next divides into the 



