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. [21] _Bull. Soc. Med. et Chirurg. Indo-Chine_, iv, p. 474. In return cases, or prolonged untreated cases of entamœbic dysentery, a generation of smaller trophozoites is associated with, or replaces the larger ones. In stools they are frequently refractile and consequently stain slowly _intra vitam_. These trophozoites are the “smaller, senile, or pre-cyst generation” of Darling. This pre-cyst generation is characterized by the presence of blocks of crystalloidal substance in the cytoplasm, and by the possession of a prominent, densely stainable karyosome. Darling believes this generation to be the same as that described by Elmassian as _Entamœba minuta_.[22] [22] _Centralbl. f. Bakter._, Orig., lii, p. 335.