It should lastly be pointed out in this connection that mixed infections also take place. For instance, in addition to _E. histolytica_, _E. coli_, and, under certain circumstances, flagellates may be found together. In the same way _E. coli_ may come under observation even in bacillary dysentery. On the other hand, Schaudinn stated that in cases of dysentery endemic in Istria, _Entamœba coli_, if it had hitherto been present, disappeared, to return again after recovery from the illness. [Illustration: FIG. 8.--_Entamœba histolytica_. _a_, trophozoite (_tetragena_ type) containing red blood corpuscles, × 1,300; _b_ and _c_, two isolated nuclei showing different appearances of karyosome, centriole and nuclear membrane, × 2,600. (After Hartmann.)] (_Entamœba tetragena_, Viereck, 1907.) This amœba must now be considered to be a part of the lifecycle of _Entamœba histolytica_, in fact a very important part of that cycle, especially in its tetranucleate cystic stages. This organism, the so-called _Entamœba tetragena_, may occur in the human intestine in cases of amœbic dysentery, especially in mild or chronic cases. It was discovered by Viereck in 1907 in patients suffering from dysentery contracted in Africa. Soon afterwards an independent description was published by Hartmann, who called the amœba _E. africana_. It was also studied by Bensen and Werner. Recently (1912–13) much work has been published on this amœba by Darling and others; in this way its relationship to Schaudinn’s _E. histolytica_ has been made known. In general morphology it somewhat resembles _Entamœba coli_, and its discoverer at first mistook it for a variety of that species. According to Hartmann, a distinct ectoplasm is only clearly visible when a pseudopodium is protruded (fig. 5). The granular endoplasm may contain ingested red blood corpuscles (fig. 6). The large, round nucleus is visible in the fresh state (fig. 8, _a_). So-called chromidial masses (? crystalloidal substances) may occur in the cytoplasm. [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.