In view of the work of recent observers, the peculiar exogenous encystment which Schaudinn made characteristic of _Entamœba histolytica_ has been shown to be due to degenerative changes in senile races of the amœba. _E. histolytica_ and _E. tetragena_ are one and the same species, and its trophozoite is subject to variation. According to some observers the _histolytica_ type of nucleus--described by Schaudinn as being poor in chromatin and not easily seen in the fresh state--occurs frequently in patients with severe symptoms of dysentery; on the other hand, the _tetragena_ type of nucleus--round and easily seen in the fresh state--may occur in cases presenting slight dysenteric symptoms. Intermediate types of nuclei are seen. The name of this species, the principal pathogenic amœba of man, must then be _E. histolytica_ by priority. The cystic stages of _E. histolytica_ are those first recorded by Viereck and formerly described as _E. tetragena_. The geographical distribution of _E. histolytica_ is wide. *Noc’s Entamœba* (1909). A species of Entamœba was cultivated by Noc[26] in 1909 from cysts derived from liver abscesses, from dysenteric stools and from the water supply of Saigon, Cochin China. He cultivated it in association with bacteria. It is pathogenic. It has been considered allied to _E. histolytica_, and shows internal segmentation or schizogony. It exhibits polymorphism. This amœba has been found by Greig and Wells (1911) in cases of dysentery in India. It is an important organism and requires further investigation. [26] Noc, F. (1909), _Ann. Inst. Pasteur_, xxiii, p. 177. Certain other Entamœbæ[27] have been described at various times from the intestinal tract of man. Probably most, if not all, of these are not good species and in some cases much more information is needed. [27] See Fantham, H. B. (1911), _Annals Trop. Med. and Parasitol._, v, p. 111. _Entamœba tropicalis_ (Lesage, 1908). This parasite is said to be non-pathogenic, and to occur in the intestine of man in the tropics. It has a general resemblance to _E. coli_, but forms small cysts (6 µ to 10 µ in diameter). The nucleus of the cyst is said to break up into a variable number of daughter nuclei, from three to thirteen having been noted. Lesage states that it is culturable in symbiosis with bacteria. It is probably a variety of _E. coli_, if not a cultural amœba. _Entamœba hominis_ (Walker, 1908) has a diameter of 6 µ to 15 µ. A contractile vacuole is present. Encystment is total, and small cysts are formed. It is culturable. The original strain, now lost, was obtained from an autopsy in Boston Hospital. This organism is probably a cultural amœba.