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Entamoeba histolytica 
seen any ingested leucocytes. When at rest it is difficult to see a well-defined 
ectoplasm, but this becomes very apparent when pseudopodia are formed, 
which are like those of the large amoebae. 
The nucleus is not visible in the living condition, but after staining it is 
seen as a small round or oval body with an average size of 4/a. In structure 
the nucleus is identical with that of the large non-dividing forms, Plate I, 
figs. 23, 24. 
Before encystment food particles disappear from the cytoplasm, and the 
animal becomes spherical or slightly oval in shape. Round the whole of the 
body a cyst wall is formed. 
Nuclear changes occur similar to those described above; the whole mass 
increases, but the karyosome becomes a little smaller and many granules are 
seen between it and the nuclear membrane, Plate VII, fig. 25. Often these 
arrange themselves in one or two semilunar masses on the membrane, Plate VII, 
fig. 26. A process of chromatin extrusion now takes place as in the tissue- 
invading forms, Plate VII, fig. 27. 
After these changes the nucleus becomes a little smaller, much of the 
peripheral chromatin disappears and the karyosome is seen as a small homo¬ 
geneous mass, on which granules are situated, Plate VII, fig. 28. During these 
processes a large vacuole developes in the cytoplasm of the cyst, Plate VII, 
fig. 26; on occasion one or two vacuoles have been seen, Plate VII, figs. 29, 30, 
and the characteristic structures known as chromidial blocks or chromatoid 
bodies make their appearance. 
The division of the nucleus is identical with that of the vegetative forms. 
The various stages are seen in Plate VII, figs. 29, 31, 32, 33. Fig. 34 shows the 
end of the process where the two daughter nuclei are connected by a thin 
chromatin thread. 
I have occasionally seen a cyst in which the two nuclei were of unequal 
size. This condition is probably brought about by the final stage of division 
being unequal, but I have never observed the process leading to this condition. 
After the separation of the nuclei they are characterised by the possession 
of a small central karyosome, composed of plastin and chromatin granules, 
and a thin nuclear membrane on which other granules are situated. The 
space between the membrane and the karyosome is also occupied by these 
bodies, Plate VII, fig. 35. Binucleate cysts are not common, probably because 
the two nuclei, soon after their formation, divide once more to form the 
characteristic quadrinucleate cyst of E. histolytica. 
This division is like that already described, but is not preceded by nuclear 
reconstruction. Plate VII, fig. 36, shows the final stage where four daughter 
nuclei are seen connected together in pairs by a narrow strand of chromatin: 
each containing a central body and granules distributed through the nuclear 
sap. After the nuclei become free in the cyst they reconstruct themselves by 
some of the granules becoming incorporated with the central body and the 
remainder passing to the nuclear membrane. Thus a quadrinucleate cyst 
