12 MANUAL OF HUMAN PROTOZOA 



to as the peripheral chromatin granules. These granules 

 are smaller than those of E. coli (p. 16-17). The most com- 

 monly observed specimens are 20-30|j in diameter. 



The trophozoites are usually found only in dysenteric 

 or diarrhoeic faeces and occasionally in the mucus appar- 

 ently eroded from amoebic ulcers voided in formed stools. 

 While the majority of the amoebae are uninucleate, there 

 may be a few binucleate ones which are undergoing divi- 

 sion that ordinarily takes place in the tissues of the colon. 



Precystic forms 



The trophozoites become transformed into cysts. The 

 transition form is known as the precystic form. Some ob- 

 servers believe that the trophozoites which occupy the 

 superficial position in the lesions of the intestine may be 

 unable to grow after division, because of lack of fresh 

 tissue cells or fluid, and remain small. Such amoebae 

 secrete cyst walls and develop into cysts. The nucleus con- 

 tains frequently larger chromatin granules and on this ac- 

 count it is not possible to distinguish E. histolijtica from 

 E. coli in this stage. 



Cysts 



1. Living specimens. The cyst (Fig. 1, 5) is nearly spheri- 

 cal in form, and its outline is almost always circular, though 

 sometimes ovoid. It is highly refractile and appears some- 

 what greenish, and therefore could be recognized with a 

 little practice under a low power objective. The homogene- 

 ous cyst wall is comparatively thin (about 0.5m thick), and 

 the protoplasm fills the space within the wall. The nuclei 

 vary in number from one to four, depending upon stages 

 of development, and are difficult to make out, although 



