368 PROTOZOOLOGY 



D. fragilis J. and D. (Fig. 173). The trophczoite is actively amoe- 

 boid; i-18fx (average 5-12/i) in diameter; progressive movement; 

 cj^toplasm well differentiated; endoplasm granulated contains bac- 

 teria in food vacuoles; nucleus onty faintly visible; 1 or 2 nuclei, the 

 ratio is variable; in some material binucleate forms may be 80% or 

 more, while in others uninucleate forms may predominate; nucleus 

 is made up of a delicate membrane and a large endosome (more than 

 one-half the diameter of nucleus) in which are embedded 4-8 chro- 

 matin granules along the periphery. According to Dobell (1940), 

 the binucleate condition represents an arrested telophase stage of 

 mitosis and the chromatin granules are in reality chromosomes, 

 probably 6 in number. Comparison with Histomonas meleagridis 

 (p. 266) led this author to think that this amoeba may be an aberrant 

 flagellate closely related to Histomonas. 





12 3 4 5 



Fig. 173. Dientamoeba fragilis, X1150 (Kudo). 1, 2, living trophozoites; 

 3, a stained uninucleate amoeba; 4, 5, stained binucleate individuals. 



Encysted stage has not been observed. Degenerating trophozoites 

 often develop vacuoles which coalesce into a large one and the or- 

 ganisms may then resemble Blastocystis hominis (p. 721) w^hich is 

 very common in faeces. Transmission may be carried on by tropho- 

 zoites. According ot Wenrich (1940), this amoeba if left in the faeces 

 remains alive up to 48 hours at room temperature, but disappears 

 probably by disintegration in 2 hours at 3.5°C. Since all attempts 

 to bring about experimental infection by mouth or by rectum failed, 

 Dobell considered that the amoeba may be transmitted from host 

 to host in the eggs of nematodes such as Trichuris or Ascaris, as in 

 the case of Histomonas (p. 266). 



The amoeba inhabits the lumen of the colon. There is no indica- 

 tion that it is histozoic or cytozoic. Some workers attribute certain 

 intestinal disturbances to this amoeba, but no definite evidence for 

 its pathogenicity is available at present. It seems to be widely dis- 

 tributed, but not as common as the other intestinal amoebae men- 

 tioned above, although in some areas it appears to be common. 



Genus Dobellina Bishop and Tate. Trophozoite: small amoeba; 

 ectoplasm and endoplasm differentiated; usually monopodial; 



