ENTAMOEBA HISTOLYTICA 33 



interpretation is supported by comparison with E. coU, in which the 

 cortical layer is more conspicuous. 



The area between the karyosome and the peripheral layer of 

 chromatin granules is normally free from chromatin. In fixed and 

 stained individuals it appears to be filled with a network of linin, often 

 with definite threads radiating from the karyosome (see figs, i, i6). This 

 "network" I regard as the optical appearance of an alveolar structure 

 probably formed by fixation. In degenerate or badly fixed individuals, 

 and in deeply-stained specimens, "chromatin" granules may sometimes 

 be seen in the linin network. But these are always, I believe, artifacts 

 or chromatin fragments or granules detached from the periphery or from 

 the karyosome. 



The nuclear characters just enumerated — the uniform layer of small 

 chromatin granules at the periphery, the central karyosome with its halo, 

 and the absence of chromatin in the intervening space — serve to dis- 

 tinguish E. histolytica with certainty from other intestinal species of the 

 genus Entamoeba. Unfortunately, one has often, in practice, to make 

 a diagnosis from organisms which are largely degenerate, and in which 

 the nuclei are abnormal ; and as it is frequently impossible in such cases 

 to determine the species of a given amoeba from the structure of its 

 nucleus, the red corpuscles in the cytoplasm often afford a more service- 

 able specific criterion of E. histolytica. 



Some writers have laid considerable emphasis on the position which 

 the nucleus occupies in the organism. It is frequently stated* that in the 

 genus Eniatnoeba the nucleus is eccentric,t whereas in the genus Amoeba 

 and other free-living forms it is central. It is true that the nucleus in 

 most entamoebae is not centrally placed : but it is not true that the 

 nucleus is usually central in species of the genus Amoeba. The belief 

 appears to me to have originated from the fact that the small free-living 

 amoebae — commonly but incorrectly referred to the genus Amoeba, and 

 wrongly called collectively "Amoeba lima.v" — usually have more or less 

 centrally placed nuclei when fixed and stained. In these amoebae, 

 however, as in all others, the nucleus constantly changes its position 

 with the movements of the organism : and it is, consequently, no more 

 typically central in them than in other forms. The central position of 

 the nucleus in a fixed and rounded small organism is, I think, an artifact, 

 which can be explained in a purely mechanical manner. It depends 

 merely upon the size of the organism and the physical state of its proto- 

 plasm whether its nucleus is or is not centralized in the process of fixa- 

 tion. At all events, in large amoebae — of any genus — this phenomenon 

 is not seen. 



Many other descriptions have already been given of the nucleus of 

 E. histolytica. There are also plenty of figures already published show- 

 ing blocks and irregular masses of chromatin on the nuclear membrane, 

 chromatin in the achromatic zone, and broken up karyosomes. Even 

 parasites without karyosomes have been described and figured. That 



* For example, see Phillips (1915): "The Entamoebae have an eccentric nucleus 

 in the resting organism instead of a centrally placed one " (p. 8) : whereas in the 

 " genus Amoeba (Ehrenberg) . . . the nucleus in the resting organisms . . . 

 is in a more or less central position " (p. 6). 



t This was, I believe, first emphasized — in the case of the intestinal amoebae of man 

 — by Schaudinn (1903). 



3 



