"GROUSE DISEASE " COCCIDIOSIS 245 



occurs and a zygote is produced. The contents of the zygote at first fill the oval 

 oocyst (PI. xxxvi., Figs. 49-52), but gradually they shrink away from 

 the poles. 1 The oocyst itself may increase slightly in size during the con- 

 centration of its contents, which ultimately form a globular mass, consisting of 

 cytoplasm rich in fatty matters, within which is a nucleus, usually centrally 

 placed. 



The nucleus (synkaryon) of the zygote proceeds to divide directly, first into two 

 (Figs. 53, 56) and then into four (Fig. 54), the divisions following one another very 

 rapidly. The granular protoplasm segments around the nuclei, and four sporoblasts 

 (Fig. 57) are produced, each sporoblast separating from its neighbours as a small, 

 rounded body (Figs. 55-57). Occasionally oocysts containing two ovoid masses of 

 protoplasm (PI. xxxvn., Fig. 82) are seen, but as a rule the form containing four 

 sporoblasts is the one found, the four sporoblasts being formed almost concurrently. 

 The sporoblasts become ovoid (PI. xxxvi., Fig. 58 ; PL xxxvn., Fig. 68), and each 

 gradually secretes a tough, chitinoid sporocyst, usually differentiated as epispore 

 and endospore, and so becomes a firm, resistant spore (PI. xxxvn., Figs. 71-76). 

 A minute amount of the cytoplasm of the zygote is not used in spore formation, 

 but remains within the zygote as a small cystal residuum. 



The sporocysts continue within the oocysts for some time, during which period 

 each sporocyst undergoes developmental changes, leading to the production of 

 actively motile sporozoites. The contents of the spore at first are homogeneous 

 (PI. xxxvn., Figs. 68-70), but gradually two refractile bodies or vacuoles appear at 

 either end (Fig. 73), and the protoplasm gradually concentrates into two masses, 

 just internal to each vacuole. The nucleus is at first central, but divides into two, 

 and the halves migrate to the opposite poles of the sporocyst (PI. xxxvi., Fig. 58). 

 The protoplasmic masses gradually displace the polar vesicles, so that the two 

 vacuoles move towards the centre and coalesce (PI. xxxvn., Fig. 73), leaving nearly 

 all the protoplasm of the sporocyst in two masses, one at each end (Fig. 71). 

 Each of the protoplasmic masses gradually becomes vermiform, extending along 

 one edge of the spore (Figs. 71, 72). Two vermiform sporozoites are thus formed 

 (Figs. 72, 74), sometimes with their more rounded ends placed at opposite ends 

 of the sporocyst (tete-beche) (Figs. 72, 74, 75), sometimes with the slightly 

 swollen ends side by side (PI. xxxvi., Fig. 59 ; PI. xxxvn., Fig. 76), the sporozoites 

 being capable of movement within the spore just previous to their escape. 

 There is a slight sporal residuum. 



1 In some cases the zygote-contents of the oocyst may be slightly nearer one pole than the other. 



