264 SPOEOZOA. 



Merozoites grow into gametocytes almost exclusively in the 

 blood- stream in the internal organs . Only matm-e gametocytes 

 are generally seen in the peripheral blood, though occasionally 

 yoiuig forms may also be found. Young gametocytes 

 are shghtly elongate, with one side concave and the other 

 convex. Fully developed gametocytes are crescentic or 

 sausage-shaped, and are generally referred to as " crescents." 

 They are about one and a half times the length and about 

 half the breadth of a red corpuscle. They usually have rounded 

 ends, but sometimes the ends are pointed, and the "crescents" 

 are then sickle-shaped. In deeply stained films the margin 

 of the blood- corpuscle is seen to be closely apphed to the 

 convex side of the crescent and stretched across the concave 

 side to show a bulge. Sometimes the crescent appears to 

 be completely surrounded by a somewhat irregular red-staining 

 border, which J. D. Thomson (1917) interpreted as a definite 

 capsule. Two types of crescents are found. The male, or 

 microgametocyte, is shorter and broader than the female, 

 and has more rounded ends ; it has hyahne cytoplasm, which 

 stains a faint blue or a faint dirty pink colour with Romano wsky 

 stains ; the nucleus is larger and paler when stained, and 

 the pigment is irregularly distributed throughout the middle 

 two-thirds of the crescent. The female gametocyte is thinner 

 and longer and has more pointed ends ; its cytoplasm is denser 

 and stains more deeply blue ; the nucleus is compact and stains 

 more intensely, and the pigment is aggregated in the centre 

 of the crescent round the nucleus. In well-stained, wet- 

 fixed fihns the nucleus of the female gametocyte is seen to 

 have a large karyosome, while that of the male has fine chroma- 

 tin granules distributed through it. The number of crescents 

 varies considerably ; sometimes a single crescent will be found 

 after a prolonged search, at other times they are so numerous 

 that one or more will be found in nearly every field. Relapses 

 are less common than in the benign tertian fever, and the 

 infection as a rule dies out more quickly. 



The Cycle in the Mosquito. — Ab in the other species of human 

 malarial parasites, the gametocytes undergo their further 

 development in the stomach of Anophehne mosquitoes. 

 There the crescents retract to a spherical form, thus causing 

 the rupture of the remains of the red blood-corpuscles, and 

 escaping out of them. Subsequent development and fertihza- 

 tion takes place as in the other species. The male gametocyte 

 gives rise to a number of microgametes by ex-flagellation, and 

 the female gametocyte forms a single macrogamete. Macro- 

 and microgametes unite in pairs, and zygotes, called ookinetes, 

 are formed, which, after penetrating the stomach-wall, 

 become lodged between the epithehum and the elastic layer of 

 the wall of the stomach. There they grow ; the nuclei undergo 



