HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



Distribution and localisation in the host. The cysts of 

 E. coli are passively carried with the food into the intes- 

 tine. Where they hatch is not known; none have ever 

 been found in the small intestine; but excystation may 

 occur there and the young amoebae may then pass on into 

 their normal habitat, the large intestine. The process of 

 excystation in the host has not been described, but the 

 writer (Hegner, 1927b) has observed the hatching of 

 coli cysts in vitro. Washed cysts either in water or in 

 weak saline solution were sealed under a cover glass and 

 placed on the stage of a microscope confined in a warm 

 chamber. The protoplasm within the cyst is at first finely 

 granular and the 8 nuclei are usually clearly visible, but 

 later the nuclei become invisible and a number of larger 

 granules of various sizes appear. The first evidence of 

 activity preceding excystation is the movement of the 

 cytoplasm in the center of the cyst. No large, free area 

 exists between the cyst contents and the cyst wall, such 

 as described by Smith (1927) in lodamoeha williamsi. 

 Pseudopodia first appear through an opening in the cyst 

 wall. This opening is small and the protoplasm streams 

 through it rapidly in a thin strand. The amoeba does not 

 leave the cyst wall at once but usually, after from one- 

 half to three-fourths of the protoplasm has escaped, 

 movement begins in the opposite direction and most or 

 all of the animal streams back again into the cyst. This 

 egress and return of the protoplasm may occur as often 

 as ten times before complete escape is efifected and the 

 liberated amoeba moves away from the deserted cyst wall. 



After excystation the amoeba moves at first slowly, 

 but soon flows across the field by means of rapidly form- 



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