8 SPOROZOA. 



a large spherical schizont with a vesicular nucleus containing 

 a large karyosome. The nucleus of the schizont divides 

 repeatedly until sixteen or thirty-two daughter nuclei are 

 produced (9, 10), and the schizont breaks up into an equal 

 number of merozoites (11). The merozoite may attack another 

 epithelial cell and undergo schizogony again, or, on entering 

 a fresh epithehal ceU, may produce a set of only four mero- 

 zoites (13-15), which differ from the others in possessing 

 a flagellum (16). The flagellated merozoites penetrate 

 other epithelial cells and become either macrogametocytes 

 (17 a) or microgametocytes (17 b). The macrogametocyte 

 grows into a large oval body containing many chromatinoid 

 and plastinoid granules, which gather round the periphery 

 to form a membrane (17 a-20 a), and thus gives rise to a 

 single macrogamete (21). The microgametocyte becomes 

 large and spherical, and within it are formed a large number 

 of biflagellated microgametes (17 6-20 6). FertiHzation (21) 

 takes place either in an epithehal cell or in the lumen of the 

 intestine. The resulting zygote (22, 23) passes out of the 

 intestine as an oocyst in the faeces (1). The protoplasm 

 within the oocyst contracts into a spherical mass (2), and 

 then divides into four spherical sporoblasts (3), which 

 become ovoidal (4), secrete a sporocyst, and become spores, 

 part of the cytoplasm not being used up in the process. Within 

 each spore two sporozoites develop (5), again leaving a small 

 amount of residual protoplasm. Fresh infection takes place 

 through the food becoming contaminated with faeces containing 

 these spores. 



While the above life-history may be taken as typical, 

 considerable variations are met with in different groups. 

 In the majority of Sporozoa the hosts are of the same species, 

 and infection is brought about by eating contaminated food ; 

 but in many forms there is an alternation of hosts, schizogony 

 taking place in an animal of one group and gametogenesis and 

 sporogony in a host belonging to an entirely different group. 

 Thus in the case of Aggregata asexual reproduction takes place 

 in a Crustacean (crab) and sexual reproduction in a Cephalopod 

 (cuttle-fish). Among the H^mosporidia schizogony takes 

 place in the blood of a Vertebrate (man, other mammals, or 

 various birds) , while sexual reproduction, followed by sporogony, 

 occurs in an Insect (mosquito). In such cases spore -capsules 

 are not formed, as the sporozoites are introduced cUrectly 

 into the blood by the mosquito. 



In the Eugregarmes the asexual cycle is entirely wantmg, 

 the sporozoite developing directly into a gametocyte. Two 

 individuals lie side by side, and an envelope or gametocyst is 

 secreted enclosing both individuals. Each individual then forms 



