lOO 



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 (li): each when full 

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



Fig. .33. — Bisexual pairing of Stylorhynchns. a, Spermatozoon ; h-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 L(5ger.) 



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 x^roduced by the schizonts, and 

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

 enters it and fuses with it (^). 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 



