GENUS: DIENTAM(EBA 249 



Dientamoeba fragilis, Jepps and Dobell, 1918. — This is a small amcBba 

 which has been seen chiefly in the unencysted stage (Fig. 116, 1-9). 

 It measures 3'5 to 1'2 microns in diameter. The amoebae are actively 

 motile, and have a well-marked ectoplasm and endoplasm. The pseudo- 

 podia are composed almost entirely of ectoplasm, and these are often 

 flattened or lobed. The endoplasm contains numerous food vacuoles in 

 which bacteria occur. The most characteristic feature of this amoeba is 

 its binucleate condition. The nuclei vary in size from 0*8 to 23 microns, 

 and are exceedingly difficult to detect in the living organism. In stained 

 specimens the nuclei are seen to be spherical, while there is a central 

 karyosome consisting of a group of granules embedded in a plastin matrix. 

 One granule is generally larger than the others. Surrounding this 

 karyosome is a clear area limited by a fine nuclear membrane, which is 

 connected with the karyosome by exceedingly delicate threads. At the 

 point of union of the latter with the nuclear membrane there are certain 

 granules, which may or may not be chromatin. Apart from these, all 

 the chromatin of the nucleus is aggregated in the karyosome. Though 

 the nuclear structure described above is characteristic of the majority of 

 amoeba? in any one case, the writer has noted that very frequently there is 

 a different arrangement of the chromatin. In some forms it appears 

 to be distributed on the inner surface of the nuclear membrane, while a 

 minute central karyosome can be detected. In other cases, chromatin 

 granules are separated from the membrane, but lie at some distance from 

 the karyosome; while in others they are concentrated at the centre of the 

 nucleus, as Jepps and Dobell state, so that the karyosome may be obscured. 

 Thomson and Robertson (1923) have called attention to the presence of 

 the central karyosome round which the chromatin granules are arranged. 

 It seems doubtful, therefore, if the aggregation of granules at the centre 

 of the nucleus should be regarded as the karyosome, which appears 

 to be represented by a minute granule, as in members of the genus 

 Entamoeba. 



Most individuals are binucleate, but a certain number of uninucleate 

 forms can generally be found. Jepps and Dobell believe that the adult 

 binucleate amoeba divides to give rise to uninucleate forms, and that as 

 these grow the nucleus divides in preparation for the division of the 

 cytopolasm, which does not take place till much later. D. fragilis is a 

 very delicate organism, and quickly degenerates outside the body. In 

 so doing, a large central vacuole often appears, reducing the amoeba to 

 a ring of cytoplasm in which the two nuclei remain. A striking resem- 

 blance to Blastocystis is thus produced (Fig. 118). 



No encysted forms were discovered by Jepps and Dobell, and the 

 writer and others have similarly failed to find any indication of encystment 



