TOXIC ACTION OF KCN ON PARAMECIUM AND DIDINIUM. 225 



mecia division is practically equal; that is, there is as much food 

 in one daughter cell as in the other. While in the case of Didinia 

 we often have unequal division, and accordingly the difference 

 in survival time is greatest at the age of one half and one hour. 

 Attention may be called to the fact that here the Paramecia 

 which were left to age in tap water and food (that is, yeast) 

 were more resistant than those left in tap water alone, and that 

 therefore the tap water as such had no deleterious effect, as might 

 have been suspected in the case of Paramecia grown in hay 

 infusion. (Cf. experiment II. above.) 



DISCUSSION. 



It is evident then from the preceding six experiments that 

 young Paramecia and starved Paramecia are more susceptible 

 to KCN than old (fed) Paramecia. In the case of Didinia, 

 although as far as average differences are concerned, the reactions 

 .of the starved and fed are like that of Paramecia, when the sur- 

 vival times of the fed animals are considered alone, it is seen that 

 there is a decided rhythm, the resistance increasing up to the 

 fourth hour and then decreasing to the sixth hour, at which time 

 a large percentage has divided. In view of this latter fact it is 

 of interest to note the similar results of Lyon and others in regard 

 to the rhythmic susceptibility of sea urchin eggs. The results 

 of Lyon's experiments (Lyon, '02) on the susceptibility of sea 

 urchin eggs to KCN solutions show roughly, although clearly, 

 that the susceptibility to KCN reaches a maximum and a mini- 

 mum at stated intervals. About ten to fifteen minutes after 

 fertilization the egg is especially suspectible to KCN. Again, 

 after the first cleavage and after the second come susceptible 

 periods with a rise in resistance in between. He was working 

 on the supposition that KCN inhibits oxidations, and so it would 

 be possible to test this further by depriving the eggs of oxygen 

 in some other way. This he did by substituting hydrogen for 

 air in the water in which the eggs were to develop. There was 

 a decided similarity between this experiment and the previous 

 one. There was a gradual loss of resistance during the first ten 

 to fifteen minutes after fertilization. Then the resistance in- 

 creased. This experiment was carried no further than thirty 



