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No immunity could be demonstrated in rabbits or guinea pigs previ- 
ously treated by intraperitoneal doses of saponin or of a saponin-serum 
mixture, although differences were observed in the susceptibility of the 
corpuscles of different animals of the same species. 
Amoebe from dysenteric patients and from tap water are destroyed by 
saponin. After a few seconds in a solution of a strength of 1 to 400 the 
ameebe burst with explosive suddenness, the ectosare disappearing, the 
granules flying in every direction, and only traces of the nucleus remain- 
ing, surrounded by a few coarse granules. . When more dilute saponin is 
used, amceboid activity increases for a few minutes; the amoeba then 
becomes more or less circular, while undulations pass around the periph- 
ery, where the clear space enlarges, both centrally (forcing the granules 
close around the central nucleus) and peripherally, the total diameter 
increasing. The granular endosarc seems to be walled in by an inner 
membrane of the ectosare, the increase in the size of the amoeba occurring 
between the two layers of the ectosarc. As the size increases, amoeboid 
motion diminishes. After a short period, varying with the concentration 
of the saponin, a change suddenly takes place in the endosare; the 
granules become larger, stream into the clear, swollen ectosare and fill 
the whole body of the ameeba as far as the limiting membrane. A few 
seconds later the amoeba either explodes in all directions, or, more com- 
monly, bursts through a pseudopod-like projection, which suddenly 
appears.” 
It was at first thought that a simple physical explanation might be found 
for the bursting of the amwbe. The property which saponins possess 
of holding solids in suspension so that they can not be separated by 
filtration has already been mentioned. It was thought, perhaps, that 
saponins would also remove ions and dissolved substances from a solution, 
perhaps holding them in colloidal suspension, in which case a simple 
osmotic pressure theory of the bursting of ameehze could be developed. 
For, as the saponin is not readily dialyzable, its power of removing dis- 
solved substances from the system (amoeba and solution) would be 
confined to the solution outside of the amceba. Hence, because of a 
resulting change in osmotic equilibrium, water would rush into the 
amoeba, in which organism more dissolved substances would be present 
than in the outside solution, until the ectosare of the amoeba would be 
ruptured explosively. ‘The experiments to decide on the value of this 
theory show that it is not the true explanation, and that the latter will 
probably need to be sought in a specific action of the saponin, possibly 
on the extosare, corresponding to the theory of the action of saponin on 
blood corpuscles as given by Stewart (12). 
2A similar action of neutral red upon ameebe has been observed by Dr. W. E. 
Musgrave. 
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