ENTAMOEBA HISTOLYTICA 3I 



which can now be used without creating confusion. The names 

 Entamoeba coli or E. dyseuteriae should in no case be employed. 

 The former can be justified, from the standpoint of nomenclature, 

 but not from that of common sense. The latter name has no 

 status in zoological nomenclature. 

 A full summary of the nomenclature and synonyms of this species 

 is provided in the list of names which heads this section (p. 20). 



Description. 



Entamoeba histolytica has been so frequently described that I shall 

 confine my description to its chief characters, and shall then discuss 

 several points of importance on which there is not yet a general agree- 

 ment among the various workers who have already described the 

 parasite. 



The active forms of E. histolytica show great variation in size, ranging 

 from about 18 /x, in diameter up to about 40 /i. As a rule, however, they 

 measure between 20 fx and 30 fi when rounded. The living animals, 

 when fresh and healthy, are extremely active. They flow along in a 

 slug-like manner with great rapidit}^ and show no conspicuous differen- 

 tiation between ectoplasm and endoplasm. When they have been out- 

 side the body of their host for some time, however, at a temperature 

 lower than that of the body, they present quite different though equally 

 characteristic movements. They then remain in one place, throwing 

 out large, hyaline, blade-like pseudopodia composed of ectoplasm 

 sharply separated from the endoplasm. This movement is, in my 

 opinion, seen only in animals which are already in some degree 

 degenerate. But it is very characteristic of this species, and serves to 

 distinguish it from E. coli. 



The endoplasm of the parasite contains numerous small granules 

 (microsomes), which may be easily stained iiitra vitam with neutral 

 red,*' flavine, and several other dyes. Apart from these granular con- 

 stituents it is homogeneous and colourless, resembling ground glass in 

 optical texture. The cytoplasm as a whole is characterized by its clear- 

 ness and freedom from inclusions. Degenerate amoebae often contain 

 bubble-like vacuoles, but these are never present in healthy individuals^ 

 They begin to appear soon after the amoebae have left the body. Food 

 vacuoles may or may not be present. When present they contain red 

 blood corpuscles, and occasionally leucocytes and fragments of other 

 cells. Normally they do not contain, in my experience, any other 

 inclusions.f Red corpuscles may be present in very large numbers. I 

 have seen an amoeba in which I could count over 40. As a rule, how- 

 ever, they are not so numerous — i to 10 being most commonly seen. 

 These ingested red corpuscles are, of course, a very characteristic feature 

 of this species : and it is probably safe to assume that any amoeba 



• It is not true, as Cutler and Williamson (1917) appear to believe, that E. histolytica 

 is the only intestinal amoeba of man which stains with neutral red intra vitam. A few 

 simple experin)ents with other species will suffice to show the fallacy of this view. 



t Wenyon and O'Connor (1917) describe and figure (Text fig. 4) specimens of E. his- 

 tolytica containing spores of a bacillus {B. megatherium ?) which they had apparently 

 ingested. 



