582 A MANUAL OF BACTERIOLOGY 



some, 0-5 JK, in diameter. The active amoebae live in the 

 tissues of the wall of the large intestine, where they 

 multiply by division. A proportion of the amoebae leave 

 the ulcers and pass into the intestinal contents, where 

 they encyst and pass out with the faeces. The precystic 

 amoebae are smaller (so-called " minuta " forms) than the 

 ordinary tissue-invasive forms. The cysts, which are 

 found only in the faeces, are smaller than the precystic 

 amoebae, and when fully developed contain four nuclei. 

 The cysts vary in size from 5 ^ to 20 /z in diameter, and 

 there are several races of the parasite, distinguished by 

 the size of the cysts. If the cysts are swallowed, they pre- 

 sumably liberate small amoebae in the small intestine 

 which pass down into the large intestine, where they 

 attack the intestinal wall and become the large histo- 

 lytica forms once more. Exceptionally, the amoebae 

 migrate to other organs, particularly the liver and occa- 

 sionally the spleen, brain and elsewhere, and in these new 

 sites they cause abscesses. The amoebae in these secondary 

 infections are always of the large tissue-invasive form. 

 E. histolytica nourishes itself at the expense of the tissue, 

 and rarely, if ever, ingests bacteria, starch grains, etc. 

 (though it may very rarely be parasitised by bacteria). 

 It probably forms a proteolytic ferment, which, when it 

 applies itself to the tissue, breaks down and dissolves the 

 cells, which melt away. The amoeba is found embedded 

 in the wall of the ulcer and abscess cavity. 



Before the amoeboid forms encyst they undergo reduc- 

 tion in size. The smaller precystic amoeba (minuta form) 

 assumes a resting stage, becomes rounded, secretes a 

 cyst wall, and thus becomes completely encysted ; this 

 occurs only in the bowel. The encysting, or encysted, 

 individual forms in its cytoplasm blocks or bars of a 

 refractile substance giving the reactions of chromatin. 

 With this formation, a vacuole also appears in the cyto- 



