AMCEB1C DYSENTERY 585 



intestine of man. It is of considerable size and averages 

 20 I.JL to 30 /x in diameter. Compared with E. histolytica, 

 its movements are sluggish and there is little differentia- 

 tion between ectoplasm and endoplasm, but the nucleus 

 is usually quite conspicuous, appearing as a round or 

 oval beaded ring and occupying an excentric posi- 

 tion (Plate XXIII, a and b). The endoplasm is 

 granular and usually contains several food vacuoles 

 with various inclusions, but not red blood corpuscles or 

 tissue elements. E. coli multiplies by binary fission. It 

 also undergoes ency station, and the precystic amoebae 

 are very similar to those of E. histolytica. The cysts 

 vary in size, and races of the parasite probably exist. 

 A cyst measuring less than 10 jit is probably not a cyst of 

 E. coli, and one measuring more than 20 //, is probably 

 not a cyst of E. histolytica. After passing through a bi- 

 and quadri-nucleate phase, the cysts usually come to 

 have eight nuclei, though occasionally six, or more than 

 eight, may be present (Plate XXIII, c). With successive 

 nuclear divisions the nuclei diminish in size, and in the 

 bi- and quadri-nucleate stage the nuclei are larger 

 relative to the size of the cyst than they are in 

 E. histolytica. E. coli is a harmless commensal in man 

 and cannot be cultivated. 



Care must be taken not to mistake the cysts of other 

 intestinal Protozoa for amoeba cysts, nor a vegetable 

 organism, Blastocystis hominis, which sometimes occurs 

 in considerable numbers in the faeces. This is more or less 

 spherical in shape and 5 to 15 /i in diameter. It is a 

 more delicate structure with thinner capsule than cysts 

 of Entamcebae. The greater part of the cyst is occupied 

 with a vacuole, the cytoplasm being reduced to a narrow 

 rim at one side, or at opposite sides, of the cysts. In 

 the cytoplasmic rim a varying number of greenish nuclei 

 are present (Plate XXIII, v). 



