CROSS-INFECTION EXPERIMENTS 87 



ent are undoubtedly also factors and it seems safe to assume that 

 symbiotic relations with bacteria and intestinal yeasts may also 

 be of importance in determining relationships. The habitat of the 

 parasite within the host also may influence rigidity, e.g., it seems 

 probable that a parasite which has adopted an intra-cellular mode 

 of existence, e.g., a coccidium, may possess a more restricted host- 

 parasite relationship than one which lives almost entirely within 

 the lumen of the intestine and penetrates only between cells, for 

 we know pH within the cell has very narrow limits while the 

 pH of the fecal contents of the intestine may vary from 8.0 

 to 5.0. 



The problems involved in carrying on investigational work on 

 the subject of cross-infection and host parasite relationships are 

 legion and the conclusions drawn as a consequence of the obser- 

 vations are related to a variety of biological fields. 



These investigations are of interest chiefly from (a) the theo- 

 retical point of view, involving questions of taxonomy and evolu- 

 tionary significance; (b) the point of view of sanitation and public 

 health involving questions of reservoir hosts which in turn are 

 dependent upon host-parasite relations; (c) the point of view of 

 experimental biology- and medicine with special reference to ex- 

 perimental pathology, histology and therapeusis. 



In order to study the extent of a disease it is important that a 

 suitable experimental animal be procured. Such procedure in- 

 volves acquainting oneself with the natural infections of the 

 animals to be employed in the experimental work, e.g., if one is 

 to attempt transfer experiments with rats and monkeys one must 

 first become thoroughly acquainted with the intestinal fauna of 

 the rat and monkey and ascertain wherein the natural protozoa 

 of the rat may dififer from those of the monkey. 



The question of a satisfactory basis for species differentiation 

 is one on which thorough agreement has not been reached by pro- 

 tozoologists. It seems to the writer, however, that such differen- 

 tiation should be based on the following determinations : 



( 1 ) Morphological or cytological dift'erences which are con- 

 stant. The chief pitfall in species differentiation among protozoa 

 on the basis of morphology is in mistaking physiological variations 

 and mitotic stages for constant morphological dift'erences. 



(2) Reactions and growth in culture. Cultural modifications 

 for species determination among the protozoa have not been 



