D. Ward Cutler 
135 
becomes constricted in the middle, Plate VII, fig. 18, and ultimately divides. 
The result of these processes is an elongate nucleus possessing at each pole a 
granular mass of chromatin together with some plastin substance, Plate VII, 
figs. 19, 20. 
Finally the nuclear membrane constricts in the middle, and ultimately 
divides into two halves, so that two small nuclei are formed, closely resembling 
that of the parent, Plate VII, figs. 21, 22. 
The nuclei now separate from each other. 
The division of the cytoplasm does not occur directly after that of the 
nucleus, for two-nucleate amoebae are commonly found and the divided 
nuclei undergo reconstruction, by a reversal of the process mentioned above, 
so that the resting type is produced before the animal divides into two. 
When this takes place the cytoplasm constricts so that two approximately 
equal animals, each containing a nucleus, are formed. I have never observed 
the division of an amoeba into more than two, and have no evidence for the 
belief that there is reproduction by schizogony, where a four-nucleate amoeba 
breaks into four small merozoites. 
The organisms produced by binary fission have two courses open to them, 
they may increase in size and become tissue invading forms, or the growth 
may be small, in which case they constitute the cyst-forming amoebae de¬ 
scribed by Elmassian as E. minuta. 
Of what determines which of these courses shall be pursued I have no 
definite knowledge. 
CYST FORMATION 
The amoeba preparing to encyst is much smaller than the tissue-invading 
form just described. Elmassian gives the size of these small animals as 12-14/z, 
but this figure is a little high, for after measuring a large number of individuals 
I find a range from 9-12-5/x, with an average size of 12-1/x. 
Dobell and Jepps (1918) with other workers have described a small 
variety of cyst measuring 10/z or less, which is identical in structure with 
the normal type. It is suggested that these small cysts are produced by a 
small strain of E. histolytica 1 . 
During the course of mv work cysts of this small type were fed to a cat, 
which developed dysentery and died. Preparations of the intestine of this 
cat showed the presence of a small variety of tissue-invading amoebae, but 
none of the normal forms have as yet been seen. I have not, however, examined 
sufficient material to be able to state that no large E. histolytica are present 
in the tissues. 
The general morphology of the cyst-forming amoeba is similar to that of the 
tissue-invading type; the endoplasm may be alveolated or granular and filled 
with food particles, consisting of blood corpuscles and bacteria, but 1 have never 
1 While this paper was passing through the press Dobell and Jepps published a detailed 
account of size variations in cysts [Parasitology , vol. x, 320). The same subject has been dealt 
with by Malins Smith (1918) in the Annals of Tropical Medicine and Parasitology , xn. 
