C. Dobell 
297 
host at its breeding time. It might therefore be supposed that the frog 
generally gets infected with E. ranarum while still a tadpole—the faeces 
of the adults infecting the young, just as in the case of Opalina. In 
support of this supposition, the fact may be adduced that tadpoles in 
nature are sometimes found to harbour E. ranarum in their intestines. 
This has been confirmed by Collin (1913). 
Taking all these facts into consideration I therefore decided to 
attempt to infect tadpoles with E. ranarum by feeding them upon 
material containing cysts of E. histolytica : since it seemed to me probable 
that it is the tadpole—not the adult frog—that normally acquires the 
infection in nature, and since it is possible to obtain tadpoles which are 
known with certainty to be-uninfected with E. ranarum before they are 
used for experiment. 
The experimental tadpoles were obtained by artificially fertilizing 
the eggs of the frog ( Rana temporaria ) in the laboratory. In order to 
ensure that no contamination of the tadpoles should occur from cysts 
of E. ranarum in or on their parents, the cultures were made in the follow¬ 
ing way 1 . The eggs and spermatozoa were removed from the frogs 
after they had been pithed. The skin was previously sponged with 
alcohol, and care was taken to avoid injury to the intestine in removing 
the genital products. The eggs were then placed in large glass dishes, 
fertilized, and covered with carefully filtered tap-water 2 . All instru¬ 
ments and dishes used were previously sterilized with boiling water. 
As an additional control, in case any of the tadpoles should subsequently 
prove to be infected with protozoa or other organisms, I made a careful 
examination of the gut-contents of every frog used in the experiments. 
None of them contained any stages of E. ranarum, though the usual 
other parasites were present in abundance ( Opalina, Nyctotherus, 
Balantidium, Eimeria, Trichomonas, Trichomastix, Octomitus, and the 
trematode Opisthioglyphe rastellus). As none of these parasites was 
subsequently found in the tadpoles in any culture, it may be concluded 
that the precautions taken were adequate. 
In this manner four cultures of tadpoles were obtained from four 
different crosses. In the first two (which I may call A and B), a large 
number of eggs hatched, producing a plentiful supply of tadpoles: but 
in the other two, only a small percentage of the eggs developed, and 
1 I am greatly indebted to Mr H. G. Newth, Demonstrator in Zoology at the Imperial 
College, for his assistance in making the cultures at a period when my time was very fully 
occupied with other work. 
2 As noted below, one "culture was made with unfiltered water. 
