The life-history (fig. 3) of the parasite exhibits two phases: (a) asexual multiplication in the intestine, either by binary fission or by schizogony with formation of eight merozoites, and (b) sporogony leading to the production of eight-nucleate cysts. Infection results from ingestion of cysts. Only cysts with eight nuclei are infective. The diameter of such cysts is about 15 µ to 20 µ.
There are varying accounts of the details of the life-cycle of Entamœba coli in its different stages. Thus, regarding schizogony or multiple fission it was formerly stated that the nucleus of the parent amœba divided into eight portions, which after dissolution of the nuclear membrane, passed outwards into the cytoplasm, which segregated around each. Eight merozoites were thus produced. More recently the process of schizogony has been considered to consist in the repeated division of the nucleus into two, four, and finally eight nuclei (fig. 3, A-D), and the formation of eight merozoites or amœbulæ.
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.
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.
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).
According to Chatton and Lalung-Bonnaire14 (1912) the entamœbæ of vertebrates should be placed in a separate genus Löschia, as they differ in their life-history from E. blattæ, the type species of Entamœba. Leidy (1879), however, named the genus Endamœba, but further researches are necessary on biological variation among these organisms.
Entamœba histolytica, Schaudinn, 1903.
Syn.: Amœba coli, autt. p. p. Amœba dysenteriæ, autt. p. p.
The average size of the amœboid trophozoite is 25 µ to 30 µ. In fæces diluted with salt solution the amœbæ swell to 40 µ and more. There is sometimes separation of the body substance into a strongly refractile vitreous ectoplasm and a corneous endoplasm, pronounced even in repose, although the former is not equally thick at all parts of the periphery. In the endoplasm generally there are numerous foreign bodies (bacteria, epithelial cells, colourless and red blood corpuscles (fig. 6), and occasionally living flagellates of the intestine). The nucleus is 4 µ to 6 µ in diameter, and may be difficult to recognize because it is sometimes weakly refractile and poor in chromatin. Its shape is slightly variable; it is usually excentric, sometimes wholly peripheral at the limit of the two parts of the body. Vacuoles are not present in quite fresh specimens, but appear later. In the study of E. histolytica, the morphological characters of the trophozoite or vegetative stage of the organism formerly separated as E. tetragena (figs. 5, 6, 8a) must be considered (see p. 38).
Fig. 5.—Entamœba histolytica (tetragena form), showing three successive changes of form due to movement. × 1100. (After Hartmann.)
The history of the development of these species, which give rise to amœbic enteritis as distinguished from bacillary dysentery, was formerly not so well known as that of E. coli. Upon being introduced into cats (per anum) dysenteric amœbæ provoke symptoms similar to those in man. In the latter, besides metastatic liver abscesses, abscesses of the lungs, and, according to Kartulis, cerebral abscesses are occasionally produced. Marchoux (1899) states that when the disease has lasted for some time liver abscesses are produced in cats also.
Fig. 6.—Entamœba histolytica which has ingested many red blood corpuscles. × 1100. (After Hartmann.)
Fig. 7.—Section through wall of large intestine (of a man) close under an ulcer caused by Entamœba histolytica. A, amœbæ that have penetrated partly in blood-vessels (Bv), partly in tissue of submucosa to the muscularis. Magnified. (After Harris.)
In the large intestine of infected cats the amœbæ creep over the epithelium, and here and there they force the epithelial cells apart, as well as removing them or pushing them in front of them; the amœbæ thus insert themselves into the narrowest fissures. They penetrate also into the glands through the epithelium, and thence into the connective tissue of the mucosa. Intestinal and glandular epithelia perish under the influence of these parasites: the cells are pushed aside, fall to pieces or are absorbed by the amœbæ. In the connective tissue of the mucosa the amœbæ migrate further, and often accumulate above the muscles. Finally they rupture this and force their way into the submucosa. In cats, apparently, the penetration is not so great as in men, according to Kruse and Pasquale. During their migration the parasites also gain access to the lymph-follicles of the wall of