340 
Races of Entamoeba histolytica 
We have already given the calculated value for the commonest size of 
cyst occurring in each of the four simple infections which we have called 
H. 8, H. 7, E. 42, B. 1. If we deal in the same way with the three double 
infections just described, we obtain the following figures for the dimen¬ 
sions of the living cysts occurring with the greatest frequency: 
Case H. 11 ... ... 8-3p, and ll-6p, 
,, E. 79 ... ... 8-3jU, and ll-6p, 
,, E. 130 ... ... 6-6/x and 11-6/a 
On comparing these figures with those already given it will be seen 
that the smaller cysts in Cases H. 11 and E. 79 have the same diameter 
(8-3 /a) as those of Case H. 7; whereas the smaller cysts of Case E. 130 
correspond in size (6-6/a) to those of Case H. 8. The larger strains from 
all these cases are, however, though coinciding with one another, different 
in size (11-6/a) from any of those which we have previously described. 
It appears, therefore, that the three double infections just described 
supply us with but one additional race. And hence we conclude that, in 
the stools of the seven patients studied, cysts belonging to five different 
races of E. histolytica were present. The approximate size of the Uving 
cysts occurring with the greatest frequency in each of these five races is 
as follows: 
6-6/a, 8-3/a, 11-6/a, 13-3/a, 15/a. 
It must be expressly noted that the strains of E. histolytica just de¬ 
scribed constitute merely a selection from those which we have studied. 
We have no doubt that, had it been possible for us to make an equally 
detailed study of all the cases in our series, we should have found other 
races possessing cysts with other mean diameters. As already noted 
above, we have found the strains with cysts having diameters from 
about 11/A to 14/a to be the commonest, though strains resembling in 
size those of Cases H. 8 and H. 7 are by no means infrequent. We have 
seen, however, very few strains indeed having mean diameters, for the 
living cysts, of intermediate magnitude (about 10/a). Of the 202 E. histo¬ 
lytica infections which we described in our earlier note (1917) there was 
but one of this character (Case E. 154)^. A case which appears to be of 
^ Unfortunately, we have been unable to make a large series of measurements of the 
cysts from this case. When the case was first examined, the size of the cysts—based on 
relatively few measurements—was recorded, and stained preparations were made for 
further study. On examining these later, however, we were disappointed to find that the 
cysts were too distorted for satisfactory measurements to be obtainable. (They were fixed 
in Schaudinn’s fluid—without acetic acid.) 
