ENTAMOEBA COLI 85 



of the cysts in these two species overlap to such an extent that only the 

 extreme limits of size are of any diagnostic value. Cysts of both species 

 may measure anything between 10 /4 and 20 /i ; but a cyst measuring 

 less than 10 fi is probably not a cyst of E. coli, and one measuring more 

 than 20 /J, probably does not belong to E. histolytica. Most English 

 workers with a large experience are now in complete agreement on this 

 point.* 



Cyst Nuclei. — The resting nuclei within the cysts of E. coli have, at 

 all stages of development, the same structure as the resting nuclei of the 

 free amoebae. The eccentric position of the karyosome, the presence 

 of chromatin granules on the linin network between it and the peripheral 

 layer, and the other slight differences noted as observable between the 

 nuclei of E. coli and E. histolytica, are constant at all stages. Far too 

 little attention has previously been paid to this fact, most observers 

 having emphasized tlie dilTerences or resemblances in the number of the 

 nuclei rather than in their structure. The structure is, however, of far 

 greater diagnostic value than the number, in most cases ; for uninucleate, 

 binucleate, and quadrinucleate stages in development are, of course, 

 common to both species. The structural differences between the nuclei 

 in the cysts of E. coli and £. histolytica will be readily appreciated by 

 comparing figs. 3, 4, and 5 with fig. 14 (PI. I), and figs. 62, 63, 66 — 69, 

 with figs, 70 — 76 (PI. IV). The position of the karyosome should be 

 specially remarked. It is typically central in every nucleus in a cyst of 

 E. histolytica, and eccentric in the nuclei of E. coli. The greatest careless- 

 ness has been shown by many workers in depicting the nuclei in the cysts 

 of both species, but especially in those of E. coli ; of which there are many 

 published figures showing central karyosomes in every nucleus in the 

 cyst — though such appearances are practically never met with in healthy 

 cysts with normal nuclei. In properly fixed and stained specimens it is 

 usually possible to determine with certainty whether a given cyst belongs 

 to one species or the other from the nuclear structure alone. 



The size of the nuclei within the cysts of E. coli is also an important 

 character. With successive nuclear divisions, the nuclei diminish in 

 size, as in the case of E. histolytica. In typical, mature, 8-nucleate cysts, 

 the nuclei are uniform in size; and they generally have a diameter which 

 is between \ and ^ of that of the entire cyst. (Cf. figs. 14 (PI. I), 62, 68 

 (PI. IV), etc.) The uninucleate cyst at the beginning of development 

 possesses typically a nucleus which has approximately twice the diameter 

 of each of the nuclei in the mature 8-nucleate cyst. The size rela- 

 tions are thus similar to those seen in E. histolytica. Consequently, in 

 binucleate and quadrinucleate cysts of E. coli the nuclei are larger — rela- 

 tively to the size of the cyst — than they are at the corresponding stages in 

 £. histolytica. This will be clearly seen by comparing figs. 4 and 5 (PI. I) 

 or 71, 73, 74, 76 (PI. IV) with figs. 59, 61, and 69 (PI. IV). It is often 

 possible to say with certainty whether a given 4-nucleate cyst belongs to 

 /i. coli or E. histolytica merely from the relative size of the nuclei (cf. 

 figs. 69 and 71) — apart from the characteristic differences in their structure. 

 Of the early stages in development of the cysts of E. coli, the 4-nucleate 

 is that least often found in the stools. Binucleate and 8-nucleate stages 



* Cf. Wenyon and O'Connor (191 7), Dobell and Jepps (191 7), Malins Smith (1918), etc. 

 The measurements here given refer to living cysts, or those examined in iodine solution. 



