HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



1923), and Karyamcebina (Kofoid and Swezy, 1924a, 

 1925a). 



II. Specific Characteristics 



I. ENDAMC^BA HISTOLYTICA 



Trophozoite. The active or trophozoite stage of E. 

 histolytica (Fig. la) varies greatly in size but is usually 

 from 20/x to 30M in diameter. The size variations are due 

 principally to two factors ( i ) growth following binary 

 division and (2) heritably diverse size races. The clear 

 ectoplasm around the periphery of the body may be dis- 

 tinguished from the more granular endoplasm; and the 

 pseudopodia, which are entirely of ectoplasm, are thin 

 and blade-like and formed in an explosive manner. 

 Within the cytoplasm are usually food vacuoles contain- 

 ing red blood cells, leucocytes or other tissue elements 

 and a single nucleus, which, however, is rarely distinctly 

 visible in the living specimens. The nuclear structure is 

 revealed in fixed and stained preparations. The nucleus 

 is of the endamoeba type (see above) ; it is from 4ju to 7m 

 in diameter; is ''poor" in chromatin; and has a small 

 centrally located karyosome and a layer of fine chromatin 

 granules on the nuclear membrane. 



Precystic stage. Before encysting, E. histolytica loses 

 its food inclusions ; decreases in size ; becomes sluggish ; 

 and rounds up. Elmassian in 1909 believed this stage to 

 be a distinct species and gave to it the name Enta/inoeba 

 minuta. Frequently vacuoles containing glycogen and 

 rod-like refractile (chromatoid) bodies appear before 

 encystation occurs. 



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