ENTAMOEBA COLI 83 



same specimen, were it not for the fact that the position occupied by 

 a nucleus in the earher is occupied by an amoeba in the later figure. 

 Except for this, and a slight difference in the position of the amoebae 

 outside the cyst, the two cysts are strikingly alike.* They both contain 

 a vacuole of a peculiar shape. So far as I am aware the presence of 

 a large nucleus in the first cyst has not been explained ; but it seems 

 inconsistent with the later account of the process of schizogony. On 

 such evidence, I am unable to believe in the "schizogony" of E. coli : 

 and I believe that any worker who has studied this species with care, 

 will, if he compares these figures with one another, and with the forms 

 usually encountered in the stools, find little to convince him of the 

 correctness of the interpretations of Mathis and Mercier. In spite of 

 the most careful search — with many other workers — I have never suc- 

 ceeded in finding cysts of E. coli liberating broods of small amoebae ; 

 and the statement that this phenomenon is " not rare " is indubitably 

 incorrect. It is so rare in fact that one may examine many hundreds of 

 stools containing E. coli, and hundreds of thousands of cysts, without 

 seeing any indication of it whatsoever. I regard the "schizogony" of 

 Mathis and Mercier as an incorrect and arbitrary series of stages, and 

 I am wholly unconvinced of the existence of this phenomenon in the 

 life-history of E. coli.-f 



Encystaiion. — Before encystation E. coli undergoes a considerable 

 reduction in size, forming precystic amoebae very closely similar to 

 those of E. histolytica. These precystic amoebae (fig. 13, PI. 1) are very 

 sluggish, or even motionless, entirely devoid of all food inclusions, 

 and consequently colourless and hyaline in appearance. They usually 

 measure from 15 /i to 18 yu, in diameter. When alive their nuclei are 

 rather more conspicuous than those of E, histolytica at the corresponding 

 stage. The karyosome is slightly larger, as a rule, and more frequently 

 eccentric; and the chromatin between it and the peripheral "ring" is 

 rather more abundant. But all these differences are very slight, and 

 not always observable. Since E. histolytica, as I have already noted, tends 

 to assume similar characters at this stage, it is, in my opinion, fre- 

 quently impossible to distinguish the precystic forms of the two species 

 with certainty — either when alive, or in the best stained preparations. 

 That these forms are often mistaken for one another I know only too 

 well from experience. I have repeatedly been perplexed and mistaken 

 in my attempts to determine them. If only these forms are present in 

 a stool, it is, in practice, unwise to pronounce a definite diagnosis. The 

 only safe course is to continue the examination of the stools until the 

 ordmary vegetative forms — with their characteristic structure and cyto- 

 plasmic inclusions — or the cysts, make their appearance. 



The precystic amoeba is doubtless formed — as in E. histolytica — by 

 division of a large organism : but the dividing forms I have never been 

 able to find. Its size also, as in this species, is proportionate to the size 

 of cyst which it will produce. 



* The second figure of " schizogony" differs also in being inverted. 



t Mercier and Mathis (1918) state that E. ranai-um^ a species apparently more 

 closely related to E. histolytica than to E. coli, also forms cysts of two sorts — schizo- 

 gonic and gametogonic. The latter are the cysts which I previously described in this 

 species, whereas the former appear to be the amoeboid forms undergoing multiple 

 fission originally discovered and described by Collin. 



