220 ROBERT H. HUTCHISON 



viously existed. But distilled water was found to have the 

 same effect, and some influence other than the salts seems to 

 be the important factor. 



Taken as a whole, the above experiments seem to point to the 

 conclusion that certain properties of the medium are important 

 factors in the heat resistance of P. caudatum, and that such 

 properties will predetermine whether a given salt solution will 

 have a favorable or an unfavorable effect. Whether this is 

 true for other Protozoa remains to be determined, and a satis- 

 factory explanation is yet to be proposed. 



EFFECTS OF ACCLIMATIZATION TO HIGH TEMPERATURES ON 



HEAT RESISTANCE 



A few experiments were carried out during the same season 

 with- a view to determining to what extent continued exposure 

 to moderately high temperatures would influence the death 

 temperature. Several attempts along this line failed, some on 

 account of too sudden changes and some for other reasons. Some 

 success was had with two cultures which were studied at fre- 

 quent intervals during an exposure of over two months to tem- 

 peratures ranging from 28° to 36°C. Pure line cultures of 

 Paramecia were used in these experiments. 



On January 30, 1913, a culture medium was prepared by tak- 

 ing 35 grams of hay in two liters of distilled water. This was 

 heated for a period of ten minutes at a temperature of about 

 60°C. This was then divided equally between two culture jars 

 so that each contained one liter of the fluid and about 17| grams 

 of hay. After cooling, each of these cultures was seeded with a 

 single individual taken from a pure line which had been growing 

 in the laboratory for 86 days before this date. The two result- 

 ing cultures, which will be referred to as '30-a' and 30-b,' may 

 therefore be regarded as of the same pure line and growing in 

 the same medium. On February 11.(12 days after the culture 

 was started) 30-a was transferred from room temperature to a 

 water bath at 28°C. The temperature of the water bath was 

 regulated automatically and never varied more than 0.5° either 

 way. The culture jar was so placed that the level of the cul- 



