TOXIC ACTION OF KCN ON PARAMECIUM AND DIDINIUM. 219 



the same, except presence or absence of food. This then supplied 

 ideal conditions for answering the question as to whether the 

 state of nutrition of the cell was the condition which determined 

 the survival time of the cell in KCN. 



Experiment IV. Didinia were cultivated in tap water in 

 Syracuse watch glasses and fed on freshly washed Paramecia 

 taken from a pure line mass culture. Animals with constrictions 

 were isolated in watch crystals in tap water, and immediately on 

 separation one sister was transferred to another watch glass in 

 which was about one c.c. tap water containing many Paramecia. 

 The two dishes, one containing one of the pair in tap water, the 

 other containing the other sister in tap water and Paramecia, 

 were then placed side by side in a moist chamber, and later the 

 two sisters were killed at the same time in the same one c.c. of 

 N/2O KCN solution. A more concentrated solution of KCN was 

 used here for convenience, to shorten the survival time, for 

 Didinium is much more resistant to KCN than Paramecium. 

 Occasionally it was impossible to tell the difference between the 

 two sisters at the end of the desired period, the one placed with 

 Paramecia evidently not having eaten; and in those cases they 

 were killed separately but as near the same time as possible- 

 By working with two binoculars it was possible to make the 

 treatment of the two with KCN practically simultaneous. 

 Thirty pairs were used in each case, so that sixty individuals 

 were killed at the end of each period of ^, i, 2, 3, 4 and 5 hours 

 after division. The results are summarized in Table IV., and 

 curves C, E and D, Fig. 2. 



The average difference in survival time (row D) increases until 

 the fourth hour, and in this lot three fed ones out of thirty-three 

 had divided; while in the next lot, that of five hours, 37.5 per 

 cent, or 1 8 out of the 48 individuals isolated had divided and 

 division of the remainder was apparently not far off. It was 

 impossible to get a record of a six hour lot on account of division. 

 Since the starved individuals did not divide at the same rate as 

 the fed ones, it was possible to continue that part of the experi- 

 ment beyond five hours. Table IV. B. It will be noted that 

 the average survival time of these starving individuals shows a 

 marked decrease. This is shown in Fig. 2, curve E. Is the 



