302 RESEARCH IN PROTOZOOLOGY 



fundamental character of distinction is the size of the oocyst. Not 

 all protozoologists are agreed that dififerences in size alone warrant 

 the naming of new species. Whether or not one size of oocysts 

 always breeds true has not been worked out in any animal except 

 the chick. 



The problem of determining the specificity of the multiple size 

 races is open to at least two methods of attack : ( i ) the production 

 of clones following inoculations with a single oocyst, and (2) the 

 correlation of size with other characteristics which pertain to the 

 race in question. 



Tyzzer (1925) working on the coccidia of gallinaceous birds 

 has devised methods for isolating and differentiating species which 

 might be readily applied to the coccidia of other animals. With re- 

 gard to dififerentiating species, the following is quoted (Tyzzer, 

 loc. cit.) : 



*Tn the recognition of species it appears to be a safer course to 

 compare various features than to depend on one alone. The char- 

 acteristics that may be taken into account are the following : 



1. The length of the period of development up to the discharge 

 of oocysts.^ 



2. The time required for sporulation at a given temperature. 



3. Host specificity or degree of restriction to a given host. 



4. Characteristic habitat or distribution of the organism in its 

 host. 



5. Cross-immunity tests. 



6. Pathogenicity as determined by both experimental and nat- 

 ural infection. 



7. Morphological studies, applying as well to the forms develop- 

 ing in the tissue and not confined solely to the size and shape of 

 the oocyst. 



8. The relation of parasite to host-cell, together with the reac- 

 tion of the latter." 



'This may be referred to as the "prepatent period." See Andrews (1926, 

 p. 787.) 



