ENTAMOEBA HISTOLYTICA 51 



having been incorrectly identified, or of their having been confused with 

 those of other species. The supernucleate cysts of E. coli, E. nana, and 

 /. biitschlii are not to be confused with the cysts of any other amoebae in 

 the same situation ; and neither the frog nor the leech is infected — so 

 far as is known — with a species of Entamoeba normally forming 

 8-nucleate cysts. 



I think it certain, therefore, that E, histolytica sometimes forms super- 

 nucleate cysts containing 8 nuclei : but this happens so very rarely that, 

 for all practical purposes, such as diagnosis, their occurrence can be 

 ignored. 



Races distinguishable by the Size of their Cysts. — As I have already 

 discussed this subject fully elsewhere in a joint paper (Dobell and Jepps, 

 1918), I shall here merely recapitulate the chief conclusions there drawn, 

 and make one or two additions. 



The fact that E. histolytica is a species which is composed of different 

 races distinguishable by the size of their cysts, was first definitely stated 

 by Wenyon and O'Connor (1917). I reached the same conclusion 

 independently, and have given the evidence for it in two previous joint 

 papers (Dobell and Jepps, 1917, 1918). In the second of these we were 

 able, I think, to place the matter beyond all reasonable doubt. It was 

 there shown, by the analysis of careful measurements of large numbers of 

 cysts from different infections, that "£. histolytica is a collective species. 

 It comprises a number of distinct races, strains, or pure lines, 

 distinguishable from one another by the size of the cysts which they 

 produce. How many such distinct races exist is still undetermined, but 

 we have demonstrated the existence of at least five. There is no 

 evidence that the different races differ in their geographical distribution, 

 or in any character save size. These races remain constant in character 

 within a given host ; and the dimensions of the cysts are not determined 

 by the action of the host upon the parasite, since two different races 

 may coexist side by side in the same host." 



The five races of E. histolytica which we specially studied had cysts 

 with mean diameters* of 6*6/*, 8*3 /*, ir6 /j,, 13*3 /x, and 15 /*. Text-fig. i, 

 here reproduced (p. 52) from our paper, will show at a glance the striking 

 differences in the dimensions of the cysts belonging to four of these 

 races (Cases H.8, H.7, E.42, B.i). The curves were plotted from 

 measurements of 500 cysts from each race, and were superposed 

 subsequently. The ordinates show the number of cysts measured, the 

 abscissae their diameters in microns. f 



Races of E. histolytica which produce cysts of the smaller sizes 

 (usually 7yx — g/j, in diameter) were long overlooked. They appear to 

 have been first recorded by Prowazek (191 2^), who found them in 

 Samoa, and regarded them as belonging to a distinct species which he 

 named " Entamoeba hartmanni." In the same year they were also found 

 by Aragao (19 12) in Brazil, and combined by him with E. coli to form 



* These figures were calculated for living cysts — the measurements having been 

 made upon fixed and stained specimens. The differences in size between living cysts 

 and those fixed, stained, and mounted in balsam, were investigated in detail, and the 

 factors determining these differences ascertained. ViWe Dobell and Jepps (1918). 



t The diameters here shown are those of stained cysts mounted in balsam, in which 

 medium the apparent diameter of a cyst is about 10 per cent, less than its true 

 diameter when measured alive in saline solution. 



