IOO 



PROTOZOA 



behaviour of the sporozoite, so that the disease spreads freely, 

 and becomes acute after several reinfections. After a time the 

 adult parasites, instead of becoming schizonts and simply forming 

 merozoites by division, differentiate into cells that undergo a 

 binary sexual differentiation. Some cells, the " oocytes " (d, e), 

 escape into the gut, and the nucleus undergoes changes by 

 which some of its substance (or an abortive daughter-nucleus) 

 is expelled to the exterior (/), such a cell is now an " oogamete " 

 or oosphere. Others, again, are spermatogones (h): each when full 

 grown on escaping into the gut commences a division (i, j), like 



Fig. 33. Bisexual pairing of Stylorhynchus. a, Spermatozoon ; b-e, fusion of cytoplasm 

 of spermatozoon and oosphere ; /, g, fusion of nuclei ; h-j, development of wall to 

 zygote ; k, I, formation of four sporoblasts ; I, side view of spore ; m, mature sporo- 

 zoites in spore. (After Leger.) 



that of the schizonts. The products of this division or segment- 

 cells are the flagellate sperms (s) : they are more numerous and 

 more minute than the merozoites produced by the schizonts, and 

 are attracted to the oosphere by chemiotaxy (p. 23), and one 

 enters it and fuses with it (g). The oosperm, zygote or fertilised 

 egg, thus formed invests itself with a dense cyst-wall, as a 

 "oospore" (k), its contents form one or more (2, 4, 8, etc.) 

 spores ; and each spore forms again one, two, or four sickle- 

 shaped zoospores (" sporozoites "), destined to be liberated for a 

 fresh cycle of parasitic life when the spores are swallowed by 

 another host. 



In some cases the oogametes are at first oblong, like ordinary 



