126 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



history falls into two parts : (i) asexual or schizogony 

 stage, passed in man ; (2) sexual or sporogony stage, 

 passed in a mosquito belonging to the genus Anopheles. 



Life-history. — (i) The Plasmodium enters its verte- 

 brate host as a minute and very slender spindle-shaped 

 creature in the saliva which mosquitoes inject when they 

 bite. It is then in its sporozoite stage. It burrows into a 

 red blood cell and gradually comes to rest within it. This 

 is the trophozoite stage. The Plasmodium becomes a 

 rounded body with a single large nucleus. A large vacuole 

 frequently develops, pushing the nucleus to one side and 

 giving a characteristic ring-like appearance (" signet-ring 

 form "). At the conclusion of the trophozoite period the 

 parasite enters on the schizont stage. Multiple fission 

 of the nucleus takes place, and the cytoplasm, save for a 

 small residual amount containing pigment granules, 

 aggregates round the daughter nuclei to form 15-20 

 merozoites. The red blood cell then bursts, and the 

 merozoites are set free in the blood. The merozoites 

 attack fresh red blood cells, and the cycle, which takes 

 forty-eight hours, is repeated. After about five cycles 

 have been passed, the amount of toxin liberated into the 

 blood from the shattering of the red blood cells sets up 

 fever in the human host, and attacks recur every forty- 

 eight hours, corresponding with the escape of the mero- 

 zoites. Eventually the second stage in the life-history 

 becomes inaugurated. Certain merozoites grow more 

 slowly than the others, and do not produce schizonts, 

 but, entering a red blood cell, become rounded oflF without 

 developing a vacuole, and after reaching their limit of 

 growth, become free in the blood as male and female 

 gametocytes, the latter somewhat larger in size than the 

 former,"but with a smaller nucleus. (2) If at this stage some 

 of the gametocyte-containing blood is sucked in by an 

 anopheline mosquito, the gametocytes alone survive 

 digestion, and sporogony begins. The male gametocytes 

 give rise to 4-8 slender active microgametes, which swim 

 rapidly about until contact occurs with a macrogamete 

 produced after maturation from a female gametocyte. 

 The tiny microgamete enters the macrogamete, their 

 nuclei fuse, and a zygote (fertilised cell) is formed. The 



