132 ROBERT H. HUTCHISON 



Do different 'races' or 'biotypes' of the same species differ from 

 each other in their resistance to high temperatures, as they have 

 been shown to do in many other respects? Under what condi- 

 tions may the resistance, of the race or species be altered? The 

 following experiments, intended to throw light on these points, 

 were undertaken at the suggestion of Dr. M. H. Jacobs~ and 

 carried out during the winter of 1911-1912 in the Zoological 

 Laboratory of the University of Pennsylvania. It is intended 

 ' to supplement the work reported here with more extensive ex- 

 periments on the influence of salts upon thermal resistance of 

 Protozoa and to learn to what extent acclimatization may be 

 produced by growth at a comparatively high temperature. 



In order to compare the resistance of different species, the 

 time element must be the same for all experiments. It is a 

 matter of common knowledge that the time of exposure has a 

 great deal to do in determining the fatal temperature. Loeb 

 says (Dynamics of living matter, p. 107): ''It is erroneous to 

 speak of a definite temperature as a fatal one, instead we must 

 speak of a deadly temperature zone For warm- 

 blooded animals 45°C. is fatal, but 42° is also fatal if maintained 

 for a longer time." Rautman ('09) gives the limits for life for 

 Paramecium as 5° to 35°C., but says that it will withstand 45° 

 if exposed for a very short time. According to Jackimoff, Tryp- 

 anosoma Brucei is killed by exposure to 45° for fiv^ minutes 

 while it resists a temperature of 43° for twenty-five minutes. 

 The time element is therefore an important factor. Preliminary 

 experiments on the determination of death temperatures with 

 the ordinary warm stage apparatus proved unsatisfactory in 

 that it was not easy to raise the temperature gradually nor to 

 bring about this gradual rise in the same length of time in suc- 

 cessive experiments. Another objection to the warm stage was 

 that the number of animals which could be kept under obser- 

 vation was very small and accurate counting was not possible 

 when large numbers were present. Moreover, the temperature 

 at which swimming movements stop is not the same as the death 

 temperature, and in using the warm stage and a binocular dis- 

 secting microscope it is not easy to distinguish temporary rigor 



