136 RESEARCH IN PROTOZOOLOGY 



activities of the host itself. The protozoa are discharged from an 

 infected host in the feces and if by chance infective stages con- 

 taminate the food or drink of another host, invasion is accom- 

 pHshed and they are taken into the nev\^ host's stomach. Here 

 they are supposed to meet a barrier — a means of protection pos- 

 sessed by the host — in the gastric juice v^ith its acidity and pro- 

 tein digestive pov^ers. No flagellate is knovi^n to colonize itself 

 in the stomach of a rat, but they do establish themselves in 

 the small intestine and cecum, hence must pass through the 

 stomach. 



The invading flagellates may succeed in passing the stomach 

 barrier either by the aid of special protective membranes (cysts) 

 or by the aid of natural pov^ers of resistance without special mem- 

 branes. ChilomasHx, Hexamitus and Giardia are known to pro- 

 duce cysts, but similar cysts are not well established for the 

 trichomonads. Hegner ( 1924) first showed that T. muris can pass 

 through the stomach of a rat and into the small intestine in a 

 viable state without encystment. This conclusion was confirmed 

 by Wenrich and Yanofif (1927) and extended to include T. parva 

 and T. minuta. The latter authors also showed that Pentatricho- 

 monas of man, which is probably the same as that in the rat, could 

 pass the stomach barrier of the rat. 



The above considerations suggest the following problems: (a) 

 for the flagellates in the vegetative state : ( I ) what conditions are 

 necessary for viability outside the host (Hegner, 1928a, lias made 

 such a study for Trichomonas of man) ? (2) is the scarcity of 

 cysts in the trichomonads to be attributed to an acquired resistance 

 to the gastric juice of the host? (3) how does this resistance com- 

 pare with that of flagellates which regularly form cysts? (4) what 

 range of hydrogen-ion concentration can trophozoites withstand? 

 (5) what range of strengths of gastric enzymes can trophozoites 

 withstand? (6) what variations in these two conditions (acidity 

 and enzyme concentration) occur in the host, i.e., can the flagel- 

 lates invade any host any time, or does invasion require special 

 conditions in the host? (7) what is the effect of sudden changes 

 of hydrogen-ion contraction on the flagellates (entrance to stomach 

 and passage from stomach to small intestine) ? (8) how are tro- 

 phozoites affected by the secretions they meet in the small intestine 

 (bile, pancreatic juice, secretions from the small intestine) ? (9) 

 what are the physiological differences among the different species 



