IX] LIFE-CYCLE OF MALARIAL PARASITE I33 



cycle. All these are digested, the macro- and microgame- 

 tocytes being the only forms that are capable of re- 

 sisting the secretions in the stomach of the mosquito. Here 

 they undergo further development, the stimulus for which 

 is probably the reduction in temperature, combined with 

 the dilution of the blood that takes place in the gut of the 

 insect. 



The macrogametocyte escapes from its red cell and becomes 

 spherical ; then its nucleus divides and one of the daughter- 

 nuclei, together with a small amount of cytoplasm, is extruded 

 as a polar bodj' (Fig. 40, 14 a). After this maturation process 

 is complete the parasite is now ready for fertilization, and is 

 known as the female or macrogamete. 



The microgametocyte undergoes a different kind of develop- 

 ment, for after becoming spherical and escaping from the red 

 corpuscle, its nucleus breaks up into a number of fragments, 

 sometimes known as chromidia, which travel outwards and 

 become arranged round the periphery of the cell. Part of the 

 nucleus remains behind at the centre as a residuum and is not 

 used up in the subsequent development. The surface of the 

 microgametocyte now grows out into a number of flagellum-like 

 processes into each of which passes one of the nuclear fragments. 

 These processes are highly motile, and by their lashing about 

 cause the parasite to resemble a flagellate. In consequence 

 the earher observers referred these forms to the genus Polymitus, 

 and they are still often referred to by this name. Each of the 

 flagellum-like processes finally becomes free and swims away 

 as an independent organism. These are the male or micro- 

 gametes. The microgametocyte usually produces four to six 

 of these microgametes which in their formation use up the 

 protoplasm of the gametocyte, merely leaving a residuum com- 

 posed of all the pigment granules, together with part of the 

 original nucleus and a small amount of cytoplasm. The 

 microgamete consists of an elongated filament with a slight 

 thickening about the middle of its length, caused by the nuclear 

 matter. It progresses by rapid wave-like movements and as it 

 is only about 0-5 ij. in diameter, is extremely difficult to observe 

 in the living state. 



