ENDAMCEBA HISTOLYTICA 133 



organism in the faeces of various animals and in sewage, but there 

 is no reason to believe that it should be considered a human para- 

 site. 



Endamceba histolytica. Endamceba histolytica, (Figs. 35 and 

 36) the dysentery ameba, is a large and active species, 18 /z to 40 M 

 (TTTHF to -$%-$ of an inch) in diameter, the majority being be- 

 tween 20 /x and 30 /z in diameter. They are rather transparent 

 in appearance, with blunt pseudopodia, and when perfectly fresh 

 from the intestine creep along in a sluglike manner. After they 

 have been outside the body for a short time they remain in one 

 place and throw out large clear pseudopodia composed of ecto- 

 plasm sharply separated from the endoplasm (Fig. 35). The 

 nucleus is very indistinct and usually eccentric in position. The 

 most characteristic feature of the living ameba, however, is the 

 presence ctf ingested blood corpuscles and tissue cells. Ordin- 

 arily there are from one to ten of these, but according to Dobell 

 there may be forty. The amebse after being fixed and stained 

 can be distinguished from the closely allied E. coli by the nu- 

 cleus. This has a peripheral layer of small uniform granules of 

 chromatin, no chromatin in the clear zone between this layer 

 and the karyosome, and a small centrally placed karyosome sur- 

 rounded by an indefinite clear halo. (Fig. 36A.) E. coli has 

 a thicker layer of peripheral chromatin, a number of chromatin 

 granules in the clear zone, and a larger karyosome, eccentric in 

 position, with a more distinct " halo " (Fig. 37A). 



There are two stages in the life history of this ameba, the vege- 

 tative and the cystic. As long as conditions in the intestine are 

 favorable for their growth and development, the amebse continue 

 in their active vegetative condition, multiplying by simple divi- 

 sion of the body into two. When conditions have become unfavor- 

 able for them, however, as in later stages of the disease, they 

 decrease in size, become rounded, and begin to develop a cyst 

 wall. This is known as the precystic stage. From this stage 

 they pass rapidly into the cystic stage by the completion of the 

 cyst wall and the division of the nucleus into four daughter nu- 

 clei, thus forming the well-known " tetragena " cysts, long sup- 

 posed to belong to a distinct species (Fig. 36B). The size of the 

 cysts varies from 5 ju to 20 M (^Vir to TTTF f an mcn ) m 

 diameter and there are a number of races or varieties of the ameba 

 which differ from each other in the average size of the cysts. 



