326 
Races of Entamoeba histolytica 
curves for the cysts in saline and iodine are also to a great extent super¬ 
imposed; but whilst the latter possesses a single summit at 13-5/x, the 
former displays two apices situated at 12/x and 13-5/x., although numerous 
cysts of intermediate size are also recorded. 
The curve obtained from the measurements in saline was so curiously 
different in character—having two apices instead of one—that we were 
at a loss to account for it. It occurred to us that a possible explanation 
might be that, in measuring the cysts, we had unconsciously tended to 
estimate the dimensions to the nearest line on the micrometer scale (12p, 
and 13-5/r represent respectively eight and nine divisions of the scale): 
and that we might thus have distributed numerous cysts of intermediate 
size to those categories immediately above and below them. We decided 
therefore to make another series of measurements from the same case. 
Accordingly, a month later, we obtained another sample of faeces from 
this patient, and measured another 500 cysts in saline. The results were 
as follows: 
Table II. 
Dimensions of cysts in units of ocular micrometer. 
7B 7-5 8-0 8-5 90 9-5 10-0 10-5 IDO 
No. of cysts 7 15 90 133 121 74 45 10 5 
These figures, when presented graphically (Fig. 3), give a curve with 
a single summit like the others—the summit in this case being at 12-75p, 
or midway between the two summits of the first saline curve (Fig. 2). As 
we have never at any other time obtained a curve of the latter type from a 
series of measurements of E. histolytica cysts, we believe that in this case 
it was an accidental result of factors which we cannot at present specify 
with precision, though we believe the explanation suggested above is 
probably correct. 
On comparing the average dimensions of the cysts measured in differ¬ 
ent media, it will be seen that the size of those measured in saline and 
iodine is approximately the same (12-99p. and 12-86p), whilst that of 
those measured in balsam is less (12-04p). We concluded therefore that 
the action of iodine has no effect upon the size of the cysts—neither 
shrinking them nor causing them to swell—when used as a fixative: but 
that the apparent size after fixing, staining, and mounting in balsam is 
appreciably diminished. This diminution in average size amounted in 
the present case to approximately 1 p. 
To what is this apparent shrinkage due? A careful study of the cysts 
in saline and balsam seemed to us to indicate quite clearly that it is due 
