E. histolytica: excystation 



of the general belief that cysts must be subjected to the 

 digestive juices in the stomach and small intestine before 

 they will hatch. The idea that cysts hatch in the small 

 intestine and not in the large intestine is also coming 

 under suspicion. Kessel (1923), for example, found 

 trophozoites of E. histolytica most commonly in the 

 cecum and none in the small intestine of rats that were 

 fed cysts of this species. Only once were they encount- 

 ered in the colon. He concludes from this that excysta- 

 tion occurs in the cecum of this animal. As regards hatch- 

 ing in the colon, Sellards and Theiler (1924), contrary 

 to the experience of Izar (1914b), have shown that his- 

 tolytica cysts injected into the large intestine of kittens 

 will excyst there provided stasis is produced and Hoare 

 (1925) found amoebae in the intestinal mucosa of a kitten 

 that had been injected rectally with material containing 

 cysts only of E. histolytica. Drbohlav (Wenyon, 1926) 

 has also noted the hatching of cysts in the large intestine 

 of kittens. Presumably excystation may also take place 

 in the large intestine of man. 



Sellards and Theiler (1924) have gone even further 

 and suggest that cysts may hatch in the colon of the same 

 individual in which they are formed. Relapse in amoe- 

 biasis may, according to this view, result from the hatch- 

 ing of resistant cysts held for long periods in the intes- 

 tine. However, the observation of Dobell and Laidlaw 

 (1926b) that cysts must be cooled below body tempera- 

 ture for several days before they will hatch indicates that 

 cysts could not hatch until they had passed out of the 

 body of the host in which they encysted. 



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