GENERA: PELOMYXA AND ENTAMCEBA 183 



The nucleus is spherical, and consists of a definite nuclear membrane, on 

 the inner surface of which the bulk of the chromatin of the nucleus is 

 distributed in the form of granules. There is a linin network, upon which 

 fine granules of chromatin may or may not occur, while a comparatively 

 small karyosome is present. Reproduction in the vegetative phase is by 

 simple binary fission, while transmission from host to host is effected by 

 means of encysted forms. The cyst is a transparent and smooth structure, 

 and the nucleus of the enclosed parasite, by repeated divisions, gives rise 

 to a number of daughter nuclei, which vary from four to multiples of four. 

 The encysted forms are passed out of the body of the host, and undergo 

 no change till they enter the intestine of another host, where the cyst wall 

 ruptures, and there is liberated either the multinucleated cytoplasmic 

 body, which then divides into a number of amoebae, or a number of amoebse 

 which have been formed before rupture of the cyst. It is not clear which 

 of these processes actually occurs. Each species of the genus tends to 

 produce a cyst which, when fully developed, contains a definite number 

 of nuclei. Thus, E. coli, E. muris, and other forms have eight, while 

 E. histolytica and E. rananim have only four. Occasionally, the nuclei 

 are in excess of the usual number. There may be sixteen or more in 

 E. coli, and more rarely eight in E. histolytica. The cysts of E. ranarum 

 may have a still larger number. This tendency to nuclear excess may occur 

 less commonly in the unencysted stages. In the case of E. coli unencysted 

 forms with eight nuclei have been described, but it is probable that these 

 were really irregularly-shaped encysted forms, the cyst walls of which, 

 in stained preparations, were not actually visible. Multinucleate free 

 forms of E. ranarum were described by Collin (1913), and similar s^^ages 

 were seen by Keilin (1917) in the case of E. ynesnili, and by the writer 

 in E. histolytica. It has been supposed by Mathis and Mercier (1917) 

 that the cysts with an abnormally large number of nuclei represent a 

 special type of multiplication by schizogony, while those with the normal 

 number are destined to give rise to gametes. They produced no con- 

 vincing evidence in support of this view. It is more probable that for 

 some reason the nuclear multiplication has continued beyond the usual 

 limits, possibly owing to excess of nutriment, or sometimes to an amoeba 

 having encysted just as it was about to divide in the free state. 



This genus includes Entamoeba coli, the harmless amoeba of the human 

 intestine; E. histolytica, the pathogenic form producing amoebic dysentery 

 and liver abscess in man; E. gingivalis, an inhabitant of the human mouth; 

 and various species which occur as intestinal parasites of animals, such as 

 E. muris of rats and mice, E. pitheci and E. nuttalli of monkeys, E. bonis 

 of cattle, E. ovis of sheep, E. testudinis of the tortoise, E. ranarmn of frogs, 

 E. minchini of the larvae of Tipulid flies, and many other species. It is 



