[Illustration: FIG. 9.--_Entamœba histolytica_ (_tetragena_ form). _a_, emission of chromatin from nucleus; _b_, nuclear division; _c_, degenerating form with two nuclei; _d_, _e_, _f_, cysts containing one, two and four nuclei respectively, and showing chromidial blocks. × 2,000. (After Hartmann.)]
Some investigators, as Hartmann,[19] lay stress on the internal structure of the nucleus (fig. 8, _b_, _c_), best seen in preparations fixed wet and stained with iron-hæmatoxylin. The nucleus is limited by a well-marked nuclear membrane, on the inside of which granules or nodules of chromatin may occur. There is a karyosome, which, in successfully stained specimens, shows, at times, a central dot called a centriole. (The nucleus of _Entamœba coli_ does not contain such a centriole.) However, the structure of the nucleus varies at different periods during the life-cycle.
[19] _Arch. f. Protistenkunde_ (1911), xxiv, p. 163.
The diameter of the trophozoites or vegetative forms (fig. 8, _a_) is variously given as from 20 µ to 40 µ. Multiplication proceeds by binary fission and also by schizogony into four merozoites.[20]
[20] _See_ Darling, 1913, _Arch. Intern. Med._, vol. ii, pl. i, fig. 3.
Reproduction takes place by endogenous encystment (fig. 9, _d_-_f_), which is preceded by nuclear division into two, reduction and then autogamy. The interpretation of the latter phenomenon as autogamy is disputed by some authors. The round cysts, which may measure 12 µ to 15 µ in diameter, contain four nuclei, together with darkly staining masses of various shapes, the so-called “chromidial blocks” (fig. 9, _f_). The cyst-wall of _E. histolytica_ (_tetragena_) is thinner than that of _E. coli_, and the diameter of the cyst is rather less. _E. histolytica_ has not yet been cultivated.
Infection in man occurs by way of the mouth by the ingestion of cysts. A patient showing acute symptoms of dysentery is not usually infective, for he is merely harbouring the large trophozoites, which, by experiment, have been shown not to be infective to animals (kittens) when administered by the mouth. The stools of recovered patients may still contain cysts, and they may thus act as cyst-carriers or reservoirs of disease by infecting water and soil. The stools of such cyst-carriers are often solid, and so cysts of _E. histolytica_ (_tetragena_) are easily overlooked. Mathis (1913)[21] points out that healthy carriers of _E. histolytica_ may be found; 8 per cent. of the natives of Tonkin examined by him were healthy carriers of cysts.