290 
Entamoeba histolytica and E. ranarum 
The Experiments. 
At first sight it might appear a simple matter to endeavour to infect 
frogs by feeding them upon material containing cysts of E. histolytica. 
It is, however, not so simple, as will be evident from the following con¬ 
siderations, which determined the method of experimentation which I 
adopted. 
If E. histolytica and E. ranarum are two forms of the same parasite, 
then the cysts of E. histolytica, when ingested by a frog, would develop 
into E. ranarum in this host; and an experimentally infected frog 
would, if subsequently examined, be found to harbour amoebae indis¬ 
tinguishable from those which might occur in it in nature. It is therefore 
necessary to be quite certain, before attempting to infect frogs by means 
of E. histolytica cysts from man, that the frogs themselves are not 
already infected with E. ranarum. Now this is a matter of great 
difficulty. We can discover that a man is infected with E. histolytica by 
examining his faeces and finding the cysts of the amoeba in them. But 
with E. ranarum and the frog the matter is very different. I have 
never succeeded in ascertaining with certainty, by examining its faeces 
for cysts, whether an adult frog is, or is not, infected with E. ranarum 
at most seasons of the year. A frog which is heavily infected—as can 
be ascertained by killing it and examining the contents of its large 
intestine—does not pass cysts as a rule in its faeces. Without killing 
the frog, therefore, it is usually impossible to discover whether it is, or 
is not, infected. 
In the course of observations extending o^er several years, I found 
stages in the encystation of the amoeba, and fully formed cysts, in the 
contents of the large intestine only during winter (when the frog normally 
neither feeds nor passes faeces) and early spring; that is to say, in the 
months immediately preceding the breeding season of Rana temporaria 
and R. esculenta in England and Germany. I have also found cysts 
of E. ranarum in the faeces passed by infected frogs during the breeding 
season, but at no other time. This fact led me to believe that the life- 
history of E. ranarum has—like that of Opalina and other parasites of 
the frog—a definite relation to the seasonal activities of its host. It 
appears to me most probable that E. ranarum lives and multiplies in 
the gut-contents of the frog during the greater part of the year, but does 
not encyst: but during the winter and early spring, when the frog 
ceases to feed and defaecate, the amoebae encyst, and are thus ready to 
escape and to infect fresh individuals when passed into the water by the 
