ECOLOGY, COMMENSALISM AND PARASITISM 393 



quickly confirmed and, as was to be expected, E. tetragena was 

 reported as a much more widely spread dysenteric ameba than 

 E. histolytica. A small precystic phase of E. histolytica was regarded 

 as a distinct species to which Elmassian (1909) gave the name 

 E. minuta. Koidzumi (1909) created a new species, E. nipponica, 

 for a variety of amebae, some of which were probably E. dysenteriae 

 (histolytica). Other synonyms, originally suggested for the most 

 part as new species, were: Entamoeba schaudinni, Lesage (1908); 

 E. hartmanni, Prowazek (1912); E. braziliensis, Aragao (1912); and 

 several others since 1912. At this time (1912) suspicions as to the 

 identity of these suggested species began to appear in the works of 

 Darling (1912), Whitmore (1911) and James (1914) which turned 

 to certainty in the work of Walker (1911) and Walker and Sellards 

 (1913) who demonstrated the identity of E. histolytica, E. tetragena 

 and E. minuta and so brought to an end this particular period of 

 confusion, and, in addition, added many important points concern- 

 ing the distribution and transmission of the organisms of dysentery. 



4. The Modern Period.— The general acceptance of the organism 

 now known as Endamoeba coli as a harmless commensal, together 

 with the proof that the organism End amoeba dysenteriae (histolytica) 

 is pathogenic to man, was the basis for a good start in the modern 

 period. 



There is little doubt that Endamoeba dysenteriae (histolytica) is 

 a dimorphic species which, in one phase, is a tissue-penetrating 

 type which, presumably by secretion of a proteolytic ferment, causes 

 cytolysis of tissue cells leading to ulcerations and abscess formation. 

 Such a ferment has been extracted by Craig from cultures. The 

 other type is the minuta form which shows a more complete adap- 

 tation to the intestinal environment of man. This is the type 

 found in carriers and, were it not for the possibility of its trans- 

 formation into the pathogenic phase, might well take its place with 

 E. coli and other harmless amebae of the intestine. It reproduces 

 by division in the intestine, however, and is regarded as the typical 

 form of the dysentery -causing ameba (Mathis and Mercier, 

 Reichenow, etc.) which under certain conditions may revert to the 

 larger pathogenic form (Kuenen and Swellengrebel, 1913). Dobell, 

 on the other hand, maintains that it is a pre-cystic condition giving 

 rise only to the encysted form with from 1 to 4 nuclei. These cysts 

 are present in the formed stools while living minuta forms may be 

 found in fluid stools or after a purgative. The dysenteric forms are 

 not ordinarily found in stools, but may be present in the discharge 

 from ulcers. In artificial culture medium the pathogenic form 

 quickly passes into the minuta phase. Successful cultures were 

 made by Cutler (1918), by Boeck and Drbohlav (1925) and with 

 remarkable results by Cleveland and Sanders (1930). The latter 

 were able not only to cultivate the organisms indefinitely and in 



