The process of encystment is initiated by the extrusion of all liquid and foreign bodies from the protoplasm, which assumes a spherical form (fig. 4, A). The rounded uninucleate amœba then secretes a soft gelatinous coat, which finally differentiates into a double contoured cyst wall in older cysts. According to Casagrandi and Barbagallo, the size of the cyst varies from 8 µ to 30 µ, and averages about 15 µ. According to Schaudinn (1903) the cytological changes during cyst formation are as follows. The nucleus of a rounded uninucleate form divides into two (fig. 4, B). Each of these nuclei fragments into chromidia (fig. 4, C), some of which are absorbed, while others reunite so that the cell becomes binucleate again. Each of these nuclei, by a twice repeated division, produces three nuclei (fig. 4, D), the smaller two of which degenerate and were regarded as reduction nuclei. There is a clear zone or vacuole in the middle of the cyst during these maturation processes, dividing the cyst into two halves. After the nuclear reduction the clear space disappears, and each nucleus (termed by some a gamete nucleus) divides into two pronuclei (fig. 4, E). The pronuclei of the pairs were said by Schaudinn to differ slightly. Copulation occurs between pairs of unlike pronuclei, and is an example of autogamy (fig. 4, F). When complete, each of the fusion nuclei (synkarya) divides twice, giving rise first to four and finally to eight nuclei. Eight amœbulæ are thus formed within the cyst. According to Hartmann and Whitmore (1911)[13], however, autogamy does not occur within the cysts of _E. coli._ They consider that eight small amœbulæ are formed (fig. 3, _2_-_10_) which escape from the cyst and then conjugate in pairs (fig. 3, _10_-_12_), afterwards growing into a new generation of trophozoites. [13] _Archiv f. Protistenkunde_, xxiv, p. 182. Only some 10 to 20 per cent. of the cysts evacuated with the fæces undergo the full course of development, the majority perish previously. In old dry fæces, only cysts with eight nuclei are found, and it is these alone that cause the infection. _Entamœba williamsi_, _E. bütschlii_, _E. hartmanni_ and _E. poleki_ (Prowazek) are probably only varieties of _E. coli_. [Illustration: FIG. 4.--So-called autogamy of _Entamœba coli_. A, rounded amœba; B, nucleus dividing; C, the two daughter-nuclei giving off chromidia; D, each nucleus has formed two reduction nuclei; E, cyst membrane formed, and gamete nuclei are dividing; F, cyst with two synkarya.] The principal feature distinguishing _Entamœba coli_ from _E. histolytica_ is the formation of eight-nucleate cysts by the former as contrasted with the tetra-nucleate cysts of the latter. The cyst-wall of _E. coli_ is thicker than that of _E. histolytica_ (_tetragena_). Further, _E. coli_ does not usually ingest red blood corpuscles, nor are “chromidial blocks” present inside its cyst (see p. 40).