THE GREGARINES AND COCCIDIA 



331 



parasite no longer, and may now be ejected with the faeces. The 

 nucleus of each sporont then divides by repeated binary fission 

 (Fig. 144, b) into a large number of nuclei, which place themselves 

 at the surface of the body (Fig. 144, c). 



A question much debated with regard to the life-history of gregarines is 

 whether a single sporont can encyst by itself, without association with another, 

 and then proceed to the formation of spores. It has been asserted frequently 

 that this can occur, and the suggestion has been put forward that the differences 

 in the size of the spore observed in some species may be correlated with double 

 or solitary encystment. Schellack (630) has discussed the question in detail, 

 and is of opinion that in septate eugregarines solitary encystment either does 



Fro. 144. Schematic figures of syngamy and spore-formation in gregarines. 

 a, Union of two sporonts in a common cyst ; 6, various stages of nuclear 

 division in each sporont ; c, formation of gametids beginning (" pearl-stage ") ; 



d, stages in the copulation of the gametes : in the left upper quadrant of the 

 figure, separate gametes are seen ; in the left lower quadrant the gametes are 

 uniting in pairs ; the right lower quadrant shows fusion of the pronuclei ; 

 and in the right upper quadrant complete zygotes (sporoblasts) are seen ; 



e, stages in the division of the nuclei of the sporoblasts, which assume an 

 oval form ; a different stage is seen in each quadrant, eight nuclei being 

 present in the final stage ; /, cyst with ripe spores, each containing eight 

 sporozoites ; two spores are seen in cross-section. Modified after Calkins and 

 Siedlecki. 



not occur, or leads to nothing if it does, but that amongst the Acephalina and 

 schizogregarines it can take place ; a clear case has been described by Leger 

 in Lithocystis schneideri, parasite of Echinocardium ; and in Monocystis 

 pareudrili solitary encystment leading to spore -formation is described by 

 Cognetti de Martiis. In some species cysts containing three sporonts have 

 been seen ; Woodcock also found a specimen of Cystobia irregularis with three 

 nuclei. With regard to the differences in the size of the spores, the possi- 

 bility has to be taken into account that in some cases they may be developed 

 parthenogenetically that is to say, the gametids may each become a sporo- 

 blast directly, without copulation with another. 



