396 BIOLOGY OF THE PROTOZOA 



by the blood to various sites in the body, it is only in rare cases that 

 organs apart from the digestive tract become infected. Brain ab- 

 scesses are very rare, and in these only the large tissue-invading 

 amebae are present and cysts are not found (Wenyon). Different 

 observers have reported similar amebae in urine, in ducts and tubules 

 of the male reproductive organs, in the lungs and even in the skin. 

 The so-called amebae described by Kofoid and Swezy (1922) from 

 the bone-marrow in cases of arthritis deformans and from degen- 

 erating lymphatic glands of Hodgkin's disease, and regarded by 

 them as End. dysenteriae, have not been taken seriously by the 

 majority of other students of the Protozoa. 



Other Amebae of the Human Intestine.— Amebae of different 

 kinds are characteristic intestinal parasites of all kinds of animals. 

 In man, apart from E. dysenteriae (histolytica) and its many form 

 changes, they are not pathogenic. Mutual adaptation has made 

 them, for the most part, harmless guests of the alimentary tract, 

 while as stated above, even End. dysenteriae in "carriers" is a 

 harmless commensal. 



Dientamoeba fragilis was discovered by Jepps and Dobell in 1918. 

 It is a minute ameba (3.5 /z to 12 /x) and very active with, charac- 

 teristically, 2 minute nuclei. It is very delicate and apparently 

 quite rare. The binucleate condition, together with the structure 

 of the nuclei and the rare occurrence of cysts (Kofoid, 1923), are 

 definite characters which distinguish it from Endameba. Opinions 

 differ as to its pathogenic character, but Dobell (1919) regards it 

 on the whole as a harmless type. 



Endamoeba coll is a common but harmless commensal in the 

 digestive tract and never becomes a tissue-invading form. For the 

 casual observer it may be easily mistaken for E. dysenteriae but with 

 abundant material and with different stages of the organism there 

 is now no excuse for such a mistake. It is larger than E. dysenteriae 

 (15 y. to 30 n), has a more transparent protoplasm so that the nucleus 

 is visible in life and contains ingested food of different kinds but 

 rarely if ever does it ingest red blood corpuscles. The relative 

 scarcity of endoplasmic granules makes the difference between 

 endoplasm and ectoplasm less noticeable than in E. dysenteriae. 

 The nucleus is larger and the endosome more conspicuous than 

 in the dysentery causing ameba. The most characteristic feature, 

 however, is the cyst with its 8 nuclei (not infrequently with 16). 

 The pre-cystic forms are somewhat smaller than the active ameba 

 but the cysts are larger than in E. dysenteriae (10 /* to 30 n), usually 

 15 /* to 20 m- 



The so-called Councilmania lafleuri of Kofoid and Swezy (1921) 

 is now generally regarded by protozoologists as referring to modified 

 or aberrant types of Endamoeba coli. 



Other amebae of the digestive tract are E. gingival it found on 



