22 THE AMOEBAE LIVING IN MAN 



found in a human stool is E. histolytica if red corpuscles are present in 

 its protoplasm. (Cf. VVenyon and O'Connor, 1917.) As the corpuscles 

 are digested in the food vacuoles, they usually appear more or less 

 eroded or fragmented when inside the amoebae, and distinctly smaller 

 than those seen in the blood or in bloody stools. Contractile vacuoles 

 are, of course, invariably absent — as in all Entamoebae. 



The nucleus -of E. histolytica is very characteristic. It has the general 

 structure typical of the genus Entamoeba, but contains less chromatin 

 than that of most other species. In the living animal it is inconspicuous 

 •or invisible, but becomes prominent and distinct as the organism dies. 

 It is vesicular and usually spherical, measuring about 4/x. to about 7 fi in 

 fixed and stained specimens. Its structure can be satisfactorily studied 

 only in amoebae which have just left the body ; for example, those 

 which have just been passed /)er a/n^//;, or which have just been scraped 

 out of an intestinal ulcer or other amoebic lesion. The nuclei of nearly 

 all the amoebae seen in a dysenteric stool, examined in the usual way, 

 are generally degenerate, — to a greater or less extent ; and most of the 

 descriptions and figures of the nuclei of E. histolytica hitherto published 

 appear to be based largely upon such degenerate individuals. 



The normal nucleus, when properly fixed and stained, always shows 

 the same structure. (See figs, i, PI. I and 16, PI. II.) It has a very 

 delicate achromatic membrane externally,* which is lined usually by a 

 single layer of small chromatin granules — thus giving the nucleus an 

 annular appearance in optical section. As a rule the granules are of 

 very uniform size, and are either in contact or only very slightly 

 separated from one another. The centre of the nucleus is occupied 

 by a small spherical karyosome, about o"5 /j, in diameter, composed, in 

 all probability, entirely of chromatin. I find no evidence that it contains 

 a centriole — as is so frequently alleged. The most carefully fixed and 

 stained preparations, examined with the best optical apparatus, invari- 

 ably show the karyosomes in all healthy normal individuals to be quite 

 homogeneous. A most important point about the karyosome is its 

 position in the nucleus. In typical healthy individuals it lies at the 

 centre. An eccentric karyosome is exceptional. (Cf. figs, i, 16.) 



In addition to the chromatic karyosome just described there is 

 frequently a more or less definite achromatic capsule-like structure 

 which surrounds it, giving to the whole the appearance of a deeply 

 stained central granule surrounded by a paler halo. This structure is 

 perhaps best seen in specimens stained by Mann's method (cf. fig. i, 

 PI. I), in which the karyosome itself is stained red, and the "halo" blue. 

 In some individuals which otherwise appear perfectly normal this 

 appearance cannot be clearly seen, and I am still undecided as to its 

 correct interpretation. I think the halo should be regarded as a part 

 of the karyosome ; but it is possibly merely a coagulum formed by 

 fixation and deposited on the chromatic part, which represents the true 

 karyosome. In my view the karyosome is probably a composite 

 structure, consisting of a central core of chromatin surrounded by a 

 more delicate cortex— the "halo" — of achromatic substance. This 



* As stated by Walker (191 1), though previously denied by Schaudinn (1903). The 

 achromatic membrane can often be easily seen in badly fixed specimens in which the 

 chromatin granules have been artificially separated from it. 



