Malarial Parasites 477 



varies — twenty-four to forty-eight hours (Plasmodium falciparum), 

 forty-eight hours (Plasmodium vivax), seventy- two hours (Plasmo- 

 dium malariae) — the schizonts mature, becoming nearly as large or 

 quite as large as the corpuscles. The pigment granules now collect 

 at the center and the substance of the parasite divides into a group 

 of equal-sized merozoits, commonly known as spores. Of these 

 there are usually eight in the meroblasts of Plasmodium malariae, 

 from fifteen to twenty-five in those of Plasmodium vivax, and from 

 eight to twenty-five in Plasmodium falciparum. As the spores be- 

 come fully formed and ready to separate, the paroxysm of the disease 

 begins. It ends as the spores are freed and enter new corpuscles to 

 begin the cycle over again. After a good many paroxysms have 

 occurred it may be observed that not all of the schizonts change to 

 meroblasts and form spores. Some remain large spheroidal bodies or, 

 as in Plasmodium falciparum, assume a peculiar crescentic form and 

 remain unchanged in the blood. These are the sexual parasites. 

 The female is usually the larger and is known as the makrogame- 

 tocyte, the male, the smaller, the microgametocyte. These are the 

 bodies which, when removed by the mosquito, lay the foundation of 

 its infection. When they are withdrawn for microscopic examina- 

 tion or exposed to the intestinal juices of the mosquito, the micro- 

 gametocyte becomes tumultuous, its granules are observed to be in 

 a state of active cytoplasmic streaming, and suddenly there burst 

 forth long slender filaments, the micro gametes or spermatozoUs. 

 These correspond with the flagella of Laveran and others, and are 

 the same bodies that Manson thought might be the form in which 

 the parasite leaves the insect's body. The microgametes lash vig- 

 orously for a time, then, breaking loose, swim away, and, as 

 MacCallum observed, conjugate with macrogametes, sexually per- 

 fect cells formed from the macrogametocytes by "reduction divi- 

 sion" and polar body formation, thus fertilizing them. As the re- 

 sult of this fertilization a zygote or ookinete is formed. It assumes a 

 somewhat elongate pointed form and attaches itself to the wall of 

 the mosquito's stomach. In the course of time it penetrates and 

 appears upon the outside, projecting into the body cavity. It 

 grows larger and rounder, divides into several segments, and even- 

 tually forms an oocyst with many small cells, which break up into 

 myriads of tiny elongate fusiform bodies, the sporozoits. These, in 

 the course of time, seem to find their way to the salivary glands, en- 

 tering into the epithelial cells and taking radial positions about the 



18, ookinete on the wall of the mosquito's stomach; 19, penetration of the gastric 

 epithelium by the ookinetes; 20 to 25, stages of sporogenesis on the outer wall 

 of the mosquito's stomach; 26, migration of the sporozoits to the salivary glands 

 of the mosquito; 27, salivary gland with sporozoits in the epithelial cells, and 

 escape of the sporozoits from the salivary glands through the insect's proboscis 

 at the time a human host is bitten; i, free sporozoit from the mosquito's saliva 

 in the human blood; 2, penetration of the sporozoit into a red blood-corpuscle, 

 beginnmg the human cycle again (Liihe). 



