ENTAMOEBA HISTOLYTICA 55 



It would be interesting to know whether cysts of different dimensions 

 belong to strains having tissue-inhabiting amoebae of correspondingly 

 dififerent sizes. Up to the present I have not been able to determine 

 this point. An attempt to infect a kitten by means of small-sized cysts 

 — made in conjunction with Dr. H. H. Dale in 1916 — was unsuccessful; 

 but the large tissue-forms could doubtless be obtained in this manner.* 

 There is no doubt, of course, that the precystic amoebae of different 

 strains correspond in size to the dimensions of the cysts which they 

 form — strains with large cysts having large precystic amoebae, and 

 strains with small cysts small precystic forms. (Cf. figs. 77-80, PI. V.) 

 But I have never yet been able to study a case, known to be infected 

 with a strain producing small cysts, in the acute dysenteric stage of the 

 infection : and with the strains producing larger cysts I have not been 

 able to estimate the size of the amoebae with sufficient accuracy to 

 render any comparison possible. 



The only relevant figures which I have been able to collect are those 

 published by Ujihara (1914), who has recorded the dimensions of the 

 active amoebae observed in the stools of a number of patients suffering 

 from acute amoebic dysentery, and the dimensions of the cysts found in 

 the faeces of the same cases subsequently. Unfortunately he gives no 

 indication of the number of specimens measured or of his method of 

 measurement — both very important points. His table shows that there 

 is no correlation between the size of the active amoebae and the size 

 of cysts which they produce. Some of the largest amoebae, in fact, 

 were found in cases which subsequently showed the smallest cysts in 

 their faeces. For example, his Case i passed, during a period of acute 

 dysentery, amoebae whose diameter is stated to have been 44-63 /t, 

 whilst the cysts from this patient measured 6'83 fx. The corresponding 

 figures from his Case 5 are 307 /i and 8-45 /i. 



I may add that my friend the late Mr. W. O. Redman King told me 

 that he had studied a mild case of amoebic dysentery which recovered 

 clinically without treatment. In the stools — of which he sent me a 

 specimen — cysts measuring 8-9 yx in diameter were present after the 

 recovery. He informed me that the amoebae passed during the 

 dysenteric attack were not noticeably smaller than those which he had 

 seen in cases infected with strains producing cysts of the larger sizes 

 (about 12 /i); but unfortunately he made no permanent preparations of 

 the amoebae, and did not measure many of those which he saw alive. 



At the present moment, therefore, the only certain conclusion which 

 can be drawn is that this question merits further investigation. 



The Early Stages of Development. Excystation. — Although the vegetative 

 amoebae and cysts of E. histolytica are now fairly well known, the earlier 

 stages of development are still undiscovered. Since Quincke and Roos 

 (1893) first put forward the suggestion, it has been amply proved that 

 man acquires his infection with the parasite by ingesting its cysts ;t but 



• Cutler (1919) has stated recently that he infected a cat by means of cysts of a "small 

 type." The animal "developed dysentery and died," and in its intestine he found "a 

 small variety of tissue-invading amoebae." It is stated that sufficient material from this 

 case has not yet been examined, but it is to be hoped that further and more precise 

 information will be forthcoming. 



+ Numerous workers have, of course, experimentally infected cats and other 

 animals by means of the cysts of E. histolytica. See p. 67 infra. Quincke and Roos 



