526 MERKEL HENRY JACOBS 
ing of the pellicle and the flowing out of droplets of clear proto- 
plasm. This may occur either before or after the cilia have 
stopped beating, but is not so constant in P. caudatum as in some 
of the other forms studied. There is reason to believe that this 
result is at least partly due to actual injury of the pellicle and not 
merely to an increase of internal pressure, since the bursting some- 
times occurs when the cell has not markedly swollen and also 
never appears until late, while the cell may reach almost its maxi- 
mum volume quite early. Furthermore, after one droplet has 
formed, thus supposedly relieving the internal pressure, others 
may form in quick succession at other parts of the cell boundary. 
This effect is not a specific one of carbon dioxide since Budgett 
(98) noted the same phenomenon in Paramecium and other pro- 
tozoa when merely deprived of oxygen in a stream of hydrogen, 
and high temperatures also cause the same result. 
The time that elapses from the beginning of the experiment 
until the death of the animal varies considerably with cireum- 
stances. The lowest average obtained in a single experiment 
was about twenty minutes, the highest over three-and-a-half 
hours, more cultures; however, approaching the latter value than 
the former. In an effort to determine to what extent P. caudatum 
can be said to have a specific resistance especial attention was 
paid to the question of the amount of variation shown by differ- 
ent individuals, races and cultures. A large mass of data was 
accumulated which will be made the basis of another paper on a 
somewhat different subject. It may not be out of place, however, 
to say here that while there is some evidence that different races 
may have different powers of resistance, these differences are 
insignificant compared with the enormous changes in resistance 
that a single race may undergo under appropriate changes in the 
culture medium. It is possible artificially to change the resistance 
very greatly, and such changes also occur naturally during the 
ageing of the culture, an old culturein general having a high resist- 
ance, and animals kept in the laboratory for a time being more 
resistant than ‘wild’ ones. Great as these variations are, however, 
they have their limits and in the dozens of cultures and thousands 
of individuals studied none were found which had as low a resist- 
