PROTOZOA 319 



is known as the asexual or schizogonic cycle, during which the plasmodia 

 multiply by the asexual method or schizogony. The second occurs in 

 the body of a mosquito and is the sexual or sporogonic cycle, involving 

 reproduction by the sexual method or sporogony. A third phase is to 

 be recognized during which the female gametocytes sporulate without 

 fertilization. This is referred to as the parthenogenetic cycle. It is 

 passed within the body of the human host and explains the recurrence 

 of malaria after more or less prolonged periods of latency. 



The Schizogonic or Asexual Cycle. — The asexual cycle begins with 

 the infection of the hmnan blood with the sporozoites by the bite of a 

 mosquito of the genus Anopheles (p. 26). The sporozoite is spindle- 

 shaped and on entering the blood at once penetrates a red corpuscle 

 where it takes a ring-like form, referred to as the signet ring stage, Liv- 

 ing at the expense of the corpuscle, the organism grows rapidly until it 

 more or less fills the corpuscle. At this stage it is known as the schizont, 

 which is the period of its ameboid movement and highest vegetative ac- 

 tivity. As the schizont matures its nucleus breaks up into a niunber of 

 daughter nuclei, each becoming surrounded by a spherical portion of 

 protoplasm to form a small reproductive element, — the merozoite, or 

 asexually formed spore. Finally the corpuscle is broken down and the 

 swarm of merozoites is liberated in the blood-plasma. Coming from the 

 same original brood, the parasites all sporulate and become liberated in , 

 the blood at the same time; this results in the constantly increasing 

 number of merozoites being liberated at stated intervals with corre- 

 sponding intervals of paroxysm in the host. The general toxic effect 

 upon the malaria patient is contributed to by the accumulated waste 

 products of the parasite's metabolism which pass into the plasma with 

 the liberation of the merozoites. Each liberated merozoite now enters 

 another corpuscle, and the asexual cycle is repeated in from twenty- 

 four to seventy-two hours according to the species of the infecting or- 

 ganism. 



This process of multiplication may continue for an indefinite time 

 or, by analogy with other parasitic Protozoa, until the vitality is ex- 

 hausted. Asexual merozoites are greatly in the majority, but certain of 

 them are potentially sexual and require a longer time to fully develop 

 into males and females when they are known as male and female game- 

 tocytes. Up to this time they are still intracorpuscular and, in the 

 estivo-autunitial or pernicious type of fever, appear as large crescents. 

 The female crescent (macrogametocyte) has numerous pigment gran- 

 ules collected in the center; the male (microgametocyte) is the mother 

 cell of the male reproductive elements (microgametes). The nucleus 

 of the male cell divides into a number of daughter nuclei which migrate 

 to the periphery and become the nuclei of the flagelliform microgametes. 

 These bodies are constantly in the blood after the first few paroxysms. 



