TOXIC ACTION OF KCN ON PARAMECIUM AND DIDINIUM. 215 



(2) some of them had also probably eaten more recently than 

 others, containing therefore more food in vacuoles. Could these 

 two factors account for the random variation from day to day, 

 and the deviation in each day's experiment? 



To answer this question I first attempted to see whether there 

 was any difference in the survival time between old and young 

 Paramecia when they were treated with N/8$ KCN solution. 

 A pure line of Paramecia was cultivated in Syracuse watch 

 glasses in hay infusion. About eight fresh watch glasses were 

 set and seeded with the same pure line every day so that a con- 

 stant supply of dividing animals was at hand. The experiment 

 consisted of four parts: 



Part I.: The watch glass cultures were looked over and animals 

 with constrictions isolated in watch crystals. As soon as one 

 divided one of the sisters was picked out with a capillary pipette, 

 washed in one c.c. tap water for about thirty seconds, and then 

 placed in N/8$ KCN solution and the death point noted; while 

 the other sister was left to age in tap water. Thirty pairs were 

 so treated, the older sisters being killed at invervals varying 

 from 25 to 215 minutes. The average survival time of the older 

 sisters was less than that of the younger sisters. 



Part II.: Thinking that the tap water might have something 

 to do with this result I repeated the experiment but eliminated 

 tap water; that is, I diluted the KCN with distilled water and 

 allowed the sisters to age in native medium (hay infusion). 

 The average survival time of the older sisters was now greater 

 than that of the younger. 



Part III.: Is this difference in result observed in parts I. and 

 II. due to the fact that the KCN was diluted with tap water in 

 one case and with distilled water in the other, or to the fact 

 that the animals aged in tap water in one case and in native hay 

 infusion in the other? To determine this I repeated the experi- 

 ment using KCN diluted with distilled water, and allowed the 

 animals to age in tap water. The average survival time of the 

 older sisters was again less than that of the younger, as in part I. 



Part IV.: I repeated the experiment, using KCN diluted with 

 tap water, and allowed the animals to age in native hay infusion. 

 The average survival time of the older sisters was greater than 

 that of the younger. These results are summarized in Table II. 



