C. Dobell and M. W. Jepps 
341 
the same type has been recorded by Wenyon and O’Connor (1917— 
Case “Kettlewell,” p. 160). Of the other cases described by these ob¬ 
servers, we note that one is similar to our Case H. 7; three others re¬ 
semble our Case E. 42; and the remaining one is of the type of our Case 
B. 1. The figures, as far as they go, of these authors thus agree closely 
with our own; and they have also concluded that “it may be safely 
stated that the majority of cases of E. histolytica infection show cysts 
with a diameter of ten to fourteen microns.” At the present time, 
however, we are not so much concerned with an investigation of the 
actual number of strains of E. histolytica which may exist, or of their 
frequency, as with the demonstration that such strains actually do exist. 
For the complete demonstration of this fact, however, it is necessary 
to prove that the mean diameter of the cysts from any patient is not 
subject to any considerable variation from day to day, but remains con¬ 
stant. That this is so we think nobody who has studied any number of 
E. histolytica infections for any length of time will be incliued to doubt. 
It is, indeed, a fact which one confirms almost daily in the course of 
routine examinations. A patient originally found to be passing cysts of 
a certain mean diameter is found later to be j)assing cysts of the same 
size whenever his stools are positive; and this applies both to untreated 
cases, and also to those who relapse after treatment with emetine. Case 
E. 42, for example, who was under observation for about seven and a 
half months, during which time he was treated and relapsed four times, 
always, when positive, passed cysts similar to those already described 
(mostly 12-14/x in diameter). We did not always measure a large number 
of cysts from his stools, but the measurements which we made were always 
closely similar. The two largest series of measurements of living cysts 
from this patient have already been recorded. Five hundred measured 
on May 21st, 1917, gave a mean diameter of approximately 13[x, five 
hundred measured on June 23rd, 1917, after a prolonged course of un¬ 
successful treatment during the period between these dates, were found 
to have a mean diameter of 13-2/z.. Similarly another case (E. 149) which 
was originally found to be passing cysts 6-5p,-8/x in diameter on January 
16th, 1917, remained under observation until the end of May. During 
this period he was unsuccessfully treated with emetine three times, and 
at each relapse cysts of the same size were present in his stools. Obser¬ 
vations similar to the foregoing could be multiplied almost indefinitely 
from our records. In fact it is a rule, to which so far we have found no 
exception, that the size of the cysts passed by any individual infected 
with E. histolytica remains constant as long as the infection persists. 
