C. Dobell and M. W. Jepps 
3*21 
Entamoeba histolytica^. We stated that there are dift'ei'ent races or strains 
of this species distinguishable from one another by the dimensions of 
their cysts, our conclusions in this respect agreeing closely with those 
of Wenyon and O’Connor (19f7). In the following pages we propose to 
present in greater detail the evidence for our conclusions. Before doing 
so, however, it is necessary to enter into some detail concerning certain 
preliminary investigations which were indispensable for a proper demon¬ 
stration of this fact. Therefore, we shall first consider these necessary 
preliminaries; then present our evidence and the conclusions which we 
have drawn from it; and finally discuss the results of some other observers 
in the light of our own findings. 
Methods and Results. 
Part I. 
A. Methods used to determine the dimensions of the cysts of 
E. histolytica. 
An exact determination of the dimensions of the cysts of E. histolytica 
is by no means as easy as it might at first sight appear. The cysts are of 
small size, seldom perfectly symmetrical, and subject to considerable 
modification in various respects by the medium in which they are 
measured. It is moreover necessary to make a large number of measure¬ 
ments before any definite conclusion can be drawn from them. We there¬ 
fore think it important to give in detail the methods we have adopted 
for this investigation. 
When the cysts are carefully examined under the microscope it is 
found that very few indeed are perfectly sphericaP. Even these which 
at first sight appear to be so, will usually be found not truly spherical if 
moved and viewed from other aspects. If we were to confine our attention 
* We continue to use this name, rather than E. dysenteriae, in referring to this organism, 
for reasons given elsewhere by one of us (Dobell, 1918). 
- Of. the outlines of cysts shown in the figures on Plate VI. We have seen it stated 
several times that the living cysts of E. histolytica are in reality perfectly spherical, and 
that fi.xed and stained cysts of different shape have been produced artificially in making 
the preparations. This may be the rule when the technique employed has been bad; but 
we find it hard to believe that anybody who has carefully studied a large number of cysts, 
both living and stained, and in many different infections, can uphold such an opinion. 
Most living cysts are not perfectly symmetrical; and obviously a.symmetrical living cysts 
are found commonly in most infections and in some almost exclusively. Although, as will 
be shown in experiments recorded below, fixation—even with a good fixative—may alter 
the shape of a cyst, it does not appreciably modify the majority: and we shall show that, 
when fixation does change the shape of a cyst, this change may actually be from an oval 
to a spherical shape. 
