HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



(Hegner, 1926c). Trophozoites from an artificial culture 

 were injected into the stomach of a guinea-pig. One hour 

 later the pig was killed and the stomach and small intes- 

 tine carefully examined. None were found in the stomach, 

 but specimens alive and moving were recovered in the 

 small intestine 6, 12, 20, 26, 28, 34, 38, 44, and 51 inches 

 posterior to the stomach. Dobell and Laidlaw (1926b) 

 report that trophozoites of E. histolytica will withstand 

 0.2 per cent HCl for 30 minutes. Whether trophozoites 

 could successfully pass through the stomach and small 

 intestine of man is doubtful, since no one has been able 

 to infect human beings with them per os, nor has any one 

 been able to infect lower animals by adding trophozoites 

 to their food. Hence, although the trophozoites are more 

 resistant than usually supposed, it seems probable that 

 infections are never brought about in nature except by 

 the ingestion of cysts. 



Are immature cysts infective f Not all of the cysts 

 passed are supposed to be infective. As noted above cysts 

 may possess i, 2, 3 or 4 nuclei when they escape from the 

 body. One patient may pass mostly uninucleate or binu- 

 cleate cysts at one time and quadrinucleate cysts at an- 

 other time, and cysts passed by different hosts may differ 

 with respect to their nuclear number. It is generally be- 

 lieved that only the quadrinucleate cysts are capable of 

 infection, and that those with i, 2 or 3 nuclei do not con- 

 tinue development outside of the body, but soon die. 

 Recently, Yorke and Adams (1926a) have shown that 

 nuclear division may occur when immature cysts are 

 placed in artificial culture media. This suggests that un- 

 inucleate and binucleate cysts that have been swallowed 



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