C. Dobell 
301 
It should be noted with regard to subculture Bo that no tadpoles were 
examined between the 6th and the 28th days after feeding. This was 
because the cysts, both in the tadpoles and in the human faeces, were 
found to be so scanty and difficult to find on the 6th day, that a more 
intensive study of this culture seemed likely to be unprofitable. Similarly 
in C 2 ; when the tadpoles were examined on the 8th day, they were found 
to contain nothing but dead cysts, though these were plentiful; and 
the culture was therefore not examined again until the 24th day, when 
all trace of cysts had disappeared. 
It is evident that the cysts of E. histolytica, even when ingested 
repeatedly, undergo no further development in the gut of the tadpole. 
They will continue to pass through the gut until they die, which may 
take as long as three weeks. They do not, when ingested, establish an 
amoebic infection in the tadpole, as they might be expected to do if they 
were identical with those of E. ranarum. All the tadpoles remained 
consistently uninfected with Entamoebae from the time when they first 
fed upon the cysts until they were completely metamorphosed. 
It appears to me probable, indeed, that the cysts of E. histolytica 
were entirely unaffected by their passage through tadpoles, and would 
have survived for the same time if they had merely been placed in water. 
I made some experiments to determine this point, with the following 
results:—In nearly every case the cysts of E. histolytica, when kept in 
a large volume of water, survived for times closely similar to those 
observed for the cysts passing through the tadpoles. Many cysts were 
generally alive at the end of a week, some at the end of two weeks, but 
most were dead at the end of three weeks. Occasionally living and 
apparently quite healthy cysts were discoverable after three weeks; 
and in a single experiment I found a few seemingly perfectly intact 
cysts, which had lain in water for five weeks after leaving the human body. 
Drying immediately kills the cysts of both E. histolytica and E. ranarum ; 
and in faeces undiluted with water it is seldom that living cysts can be 
found after the lapse of about two weeks 1 . 
In my experiments I judged the cysts to be dead when they showed 
disintegration and vacuolation of the protoplasmic contents, both in 
the fresh condition and after fixation and staining. At times also I 
used the “eosin test 2 ,” but not as a rule. Its results were consistent 
and confirmatory, but its employment seemed to me unnecessary. 
1 These findings agree on the whole with earlier records; cf. Dobell (1909), Kuenen 
and Swellengrebel (1913), Wenyon and O’Connor (1917). 
2 Cf. Kuenen and Swellengrebel (1913), Wenyon and O’Connor (1916). 
Parasitology x 
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