C. Dobell and M. W. Jepps 
343 
Since it appears probable, from these observations, that a change in 
the environment of the amoebae—in the above case, a change pro¬ 
duced by the administration of emetine to their host—may modify the 
size of the cysts which they produce, the suggestion might be made that 
the differences in size in the cysts from different patients may be due to 
comparable causes. It might be supposed, for example, that when one 
patient passes cysts with a diameter of 7 jx, whilst another passes cysts 
of 12 jjL, this is not because they are infected with two different strains, 
but because the same strain, when living in two different environments 
(hosts), produces cysts of two different sizes. Unless such a possibility 
could be excluded, it is clear that we should not be completely justified 
in speaking of the different-sized cysts, in the infections which we have 
studied, as belonging to different strains or races of the parasite. It has 
not been possible to test this supposition by direct experimental infection 
of human beings by means of cysts of known sizes; but the question can 
nevertheless be answered almost conclusively, we believe, from some of 
the observations which we have already recorded. We think that we 
have demonstrated above that cysts of two different mean sizes may 
coexist in the stools of the same host (cp. Cases E. 130, E. 79, H. 11, 
supra ); and this fact clearly indicates that the size of the cysts, whether 
large or small, is not determined by the host. It is of course conceivable, 
though we think highly improbable, that the site of infection in the large 
intestine might influence the size of the cysts produced by the parasites. 
But since we know no facts or analogies to support such an assumption 
it seems unnecessary to discuss it further. It seems to us, therefore, that 
we are justified in concluding (1) that the differences in mean diameter 
of the cysts passed by different persons are to be correlated with differ¬ 
ences in the parasites themselves and not in their hosts; and (2) that 
these differences indicate that there are distinct groups of individuals 
among the parasites themselves which may properly be designated 
“strains” or “races” ^vithin the single species E. histolytica. 
In concluding this section we may refer to a point of some practical 
importance connected with the recognition of the different strains of 
E. histolytica. It has been our custom for a considerable time to record 
the diameter of the cysts for every infection which we have studied. 
We have thus known with some degree of certainty with what strain or 
strains of E. histolytica each patient has been infected before beginning 
treatment with emetine. This knowledge has on several occasions 
proved useful subsequently. In dealing with a large number of military 
patients it is often difficult, and sometimes impossible, to make certain 
Parasitology x 23 
