230 BARBARA LEE LUND. 



4. The cessation of movement of Paramedum is a less variable 

 "end point" than cytolysis for determining the death point in 

 KCN solutions. 



5. Paramecia can be cultivated in pure line mass cultures .in 

 tap water when fed compressed yeast. In this way the chemical 

 composition of the medium and food can be kept more constant 

 than has been possible previously in work on Paramedum. 



6. Resistance of Paramedum to KCN when allowed to feed on 

 bacteria shows a marked increase, and when fed on yeast the 

 resistance increases to a smaller degree from the time of division 

 up to the following division. The resistance of Didinia to KCN 

 when fed with Parameda increased from the time of completion 

 of division until a maximum was reached some time previous to 

 the second division, and then gradually fell before the second 

 division. This rhythm is directly comparable to that found by 

 Lyon and others on echinoderm eggs. 



7. When Paramedum and Didinium are prevented from ob- 

 taining food the resistance to KCN gradually decreases below 

 its value at the completion of division. 



8. Starvation of sister cells of Didinia results in a decrease in 

 difference of survival time in KCN. This original difference in 

 sisters is apparently due to the fact that the food content is not 

 always distributed equally to the daughter cells at division. In 

 Parameda the distribution of food is practically equal, and here 

 the average difference in survival time between sisters is the same 

 immediately after division as it is after a period of twenty-four 

 hours. This small observed difference is however within the 

 limits of experimental error. 



9. The difference in resistance of fed and starved Parameda 

 and Didinia and the rhythmic change in resistance in the case 

 of Didinia between cell divisions which is closely similar to that 

 found in echinoderm eggs can at present be best explained by 

 assuming that it is due to change in permeability of the cell. 

 Penetration of KCN into the cell and hence its toxic action as 

 measured by survival time, is dependent upon the degree of 

 permeability of the cell at different times. On this assumption 

 rhythm in susceptibility depends primarily upon rhythm in 

 permeability. The possible relation between rate of intracellular 



