156 PLASMODIUM VIVAX [CH. 



From the thirtieth to the forty-eighth hour the schizont 

 divides up into from 15 to 20 merozoites, or rarely as many as 

 24. The stages in this process have been described above, 

 but it may be weU to repeat that all the melanin, together with 

 a certain amount of waste protoplasm is left unused. The 

 melanin granules are packed together in the form of one or two 

 conspicuous brown masses, usually situated at the middle of 

 the parasite. 



The merozoites are somewhat irregularly arranged, often 

 in two concentric circles ; rosette forms, such as occur in 

 P. malaricB and P. falcipavitm, are absent. 



About the forty-eighth hour the wall of the red cell bursts 

 open and the contained merozoites together with the melanin 

 and other waste products are liberated in the serum. The 

 cycle of schizogony therefore occupies 4S hours, and in conse- 

 quence the febrile attacks recur after this interval. 



In P. vivax the gametocytes are common in the peri- 

 pheral circulation as well as in the internal organs, and con- 

 sequently their development can be followed without much 

 difficulty. The macrogametocytes are large spherical bodies 

 about 12-16 M in diameter, and in addition to their size may 

 be distinguished from the schizonts by their denser protoplasm 

 and larger and more numerous melanin granules. The nucleus 

 is usually situated at the periphery and is an oval structure 

 containing numerous chromatinic granules. The microgameto- 

 cytes are similar in shape to the macrogametocytes, but are 

 much smaller (9-1 1 /.t), and in addition maybe distinguished 

 from the latter by their large densely staining nucleus. In 

 addition, the cytoplasm is \'ery much lighter and less granular. 



Both the kinds of gametocytes originate from ordinary 

 merozoites, but during their development can be distinguished 

 from the schizonts by their much slower growth and absence of 

 acti^'e amceboid and ring forms. 



The de^'elopment of P. vivax in the mosquito was first worked 

 out in Anopheles macuUpennis, one of the common mosquitoes 

 of Europe, but many other species are known to transmit the 

 infection equally well. When ingested by the mosquito, the 

 development of the gametes will not take place below 16° C, 



