ENTAMCEBA COLI 215 



teristic of the mature cyst (Fig. 101). Occasionally, a further division 

 will take place, giving rise to sixteen nuclei. The cysts with sixteen 

 nuclei, though uncommon, are much more frequently encountered than 

 the eight-nuclear cysts of E. histolytica. Very rarely, cysts with a larger 

 number of nuclei occur. During the process of nuclear multiplication some 

 of the nuclei may cease to divide, so that an irregular number of nuclei 

 of unequal size may result. According to Dobell (1919), soon after encyst- 

 ment, a glycogen vacuole forms in the cytoplasm, and this reaches its 

 maximum development at the two-nuclear stage. After this it gradually 

 shrinks till at the eight-nuclear stage it has disappeared. In the writer's 

 experience the precystic amoebse themselves may possess a large vacuole 

 or a series of vacuoles which run together after encystment has occurred. 

 This vacuole, however, is not always present. 



The cysts of E. coli vary in diameter from 10 to 30 microns. They 

 usually measure from 15 to 20 microns, but larger ones may occur, as 

 recorded by the writer and O'Connor (1917), who saw one measuring 

 38 by 34 microns. The commonest type of cyst met with in the stool is 

 one containing a clear cytoplasm in which are embedded the eight nuclei 

 (Fig. 101, 4 and 8). 



In most cases there occur also a smaller number of cysts of a different 

 type. These are usually larger than the ones just mentioned, and have 

 a large central glycogen vacuole which reduces the cytoplasm to a thin 

 layer lining the cyst wall (Fig. 101, 10-12). There are usually two 

 nuclei, which generally lie at opposite poles of the cyst. They often 

 appear as if flattened against the cyst wall by pressure of the vacuole. 

 In other cases the vacuole is smaller, and there is a thicker layer of cyto- 

 plasm. The vacuole contains glycogen, which stains brown with iodine 

 (Plate II., 2, p. 250). More rarely cysts of this type may be seen with 

 four nuclei, and still more rarely with eight nuclei. A modification is 

 occasionally seen in which a series of vacuoles occurs round the periphery 

 of the cyst (Fig. 101, 14), while the cytoplasm with the two, four, or eight 

 nuclei may occupy its centre. In optical section such cysts have a cart- 

 wheel appearance. Dobell (1919) considers that the vacuole occurs in the 

 normal course of development, and reaches its maximum size at the two- 

 nuclear stage, and that it then disappears. It seems to the writer, how- 

 ever, that these two-nuclear cysts with the large vacuole have an abnormal 

 appearance, and it is difficult for him to believe that the eight-nuclear 

 cysts with their particularly clear cytoplasm have been developed from 

 the coarsely vacuolated binucleated cysts, which, moreover, are usually 

 larger than the eight-nuclear cysts present at the same time. On a few 

 occasions the writer has examined material containing precystic amoebse 

 with perfectly clear cytoplasm, and has seen in the same material cysts 



