Immunity of Loiuer Organisms to Ethyl Alcohol 595 



(following the direction the organism is moving) until it has con- 

 sumed the entire body— leaving in its path only a granular mass 

 of disintegrated protoplasm. Death is that unmistakable point 

 at which the last trace of form is destroyed and at which the last 

 cilium can be clearly seen to stop moving. 



The fact that this organism can be obtained in large numbers 

 and kept for long periods of time, together with the fact that it 

 gives so definite a death-point, makes it in some respects prefer- 

 able even to Stentor for work of this sort. 



The Spirostoma of the following studies grew in a medium in 

 which the decayed vegetable material had settled to the bottom, 

 leaving the organisms in a relatively clear liquid. In this they 

 flourished in great abundance, furnishing most satisfactory mate- 

 rial for comparative studies for months at a time. 



Individual variation in resistance to chemicals is in the case of 

 Spirostomum most marked. Animals from the same culture, 

 when tested to a low fatal dose, may be divided roughly into two 

 groups — one dying within two to three minutes, the other (a 

 smaller group) surviving in the same fluid a much longer time. 



In work upon this form the larger group above mentioned has 

 been taken as the more representative and organisms with a resist- 

 ance exceeding seven minutes have not been counted. This 

 would seem likely to produce error, for if animals with a normal 

 resistance approaching seven minutes be raised by acchmatiza- 

 tion to or above this maximum they would be rejected as abnor- 

 mal. Those of the control, on the other hand, would not thus 

 increase above this maximum and would therefore be counted. 

 The elimination of the one and the counting of the other would 

 result in a ratio between the two that would indicate less immunity 

 than was actually present. 



As a matter of fact error of this sort has been largely controlled 

 in the following work. This was done by the use of a stronger 

 kilHng fluid, which shortened the period of resistance. In an 

 8 per cent killing fluid it was found that only a few lived longer 

 than seven minutes. 



With these considerations we may now pass directly to the 



experiments. 



