ENTAMOEBA COLI 79 



Apart from the cytoplasm and its inclusions — which will be referred to 

 again later — E. coli can be recognized most readily by its nucleus. 



The nucleus of E. coli, like that of all other Entauioebae, contains 

 most of its chromatin in a thin peripheral layer and a comparatively 

 small karyosome. Its structure is shown in figs. 12 (PI. I), 17 (PI. II), 

 and 55 (PI. IV). It is typically round or slightly oval, and its diameter 

 is usually from about 4/x, in the smallest individuals, up to about 8 /i in 

 the largest. In structure it is closely similar to the nucleus of E. his- 

 tolytica, already described ; liut it differs in the following features. The 

 external achromatic membrane is slightly thicker : the layer of chromatin 

 within it consists of rather larger granules, more closely set together, 

 though equally evenly disposed : the chromatic part of the karyosome 

 is relatively somewhat larger — measuring about i fi in good-sized indi- 

 viduals : the cortex — or " halo " — of the karyosome is more definite and 

 solid in appearance : the karyosome is nearly always eccentric — not 

 central : and definite chromatin granules are generally present in the 

 area between the karyosome and the peripheral chromatin layer. These 

 distinguishing characters will be readily seen on comparing figs, i and 12 

 (PI. 1) and 16 and 17 (PI. II). They are quite constant in perfectly fresh 

 specimens which have been properly fixed and stained : but is is fre- 

 quently impossible to distinguish E. coli from E. histolytica by its nuclear 

 characters in poor preparations, or when the organisms are in the least 

 degenerate. 



Most of the published descriptions and figures of E. coli have been 

 drawn, apparently, from a study of more or less degenerate amoebae. 

 The large, discrete masses of chromatin so often shown at the periphery 

 of the nucleus are only seen in degenerate organisms. The "ring" 

 is really, in a normal animal, very uniform. The karyosome, to which 

 too little attention has generally been paid, is rarely described in its 

 normal form. It breaks up and disappears altogether in dying or dead 

 organisms : and this accounts for the fact that in many figures it is 

 depicted as an irregular mass of granules, or is conspicuous by its 

 absence. Too little attention has also been paid to its position. In 

 the nuclei of the vast majority of individuals it is not central, though 

 often depicted in this position.* The presence in E. coli of chromatin 

 granules in the zone between the karyosome and the peripheral layer is, 

 in my experience, constant ; and the importance of this character — as 

 distinguishing the species from E. histolytica — has not been sufficiently 

 emphasized, I think, because chromatin granules are so commonly 

 seen in this position in the degenerate nuclei of the latter species. 

 In E. coli, moreover, the peripheral chromatin layer is usually very 

 resistant,! though the other constituents of the nucleus are not ; and it 

 may often be seen as a distinct ring of beads in an unfixed organism 

 which has l^een dead even for days. Dead individuals of E. histolytica 

 break up much more rapidly, and their nuclei disintegrate usually 

 within an hour or two at most. 



• In a certain number of individuals the karyosome will appear to be central, of 

 course, even if really eccentric — if the nucleus is viewed from the pole towards which 

 it is displaced. 



f And consequently, preparations of E. coli, showing fairly respectable nuclei, can 

 often be made from stale material in which all the amoebae have long since died. 

 It is unnecessary to emphasize the errors which may result from examining such 

 preparations. 



