264 Pub. Puget Sound Biol. Sta. Vol. 2, No. 48 



The cyanides, at least in most cases, apparently kill by their action on 

 respiration, whatever the exact nature of that action. In most organisms, 

 if not in all, respiration is very greatly decreased before death occurs, but 

 this decrease, if not carried too far, is completely reversible. Hyman 

 (1919) has shown, for example, that in Planaria a reversible decrease of 

 80 — 90% in oxygen consumption may occur in certain concentrations of 

 KNC, and I have found a marked decrease in COg production in the same 

 species (Child, 1919b). Very similar effects of cyanide on respiration 

 have been obtained with various other animals and plants. It is difficult to 

 understand how cynaide can produce an effect so great and at the same 

 time completely reversible, unless the protoplasm of the forms used for 

 experiment is highly permeable to it, but if such high permeability to 

 cyanide exists, differences in permeability can scarcely be entirely respon- 

 sible for the differences in susceptibility which cyanide shows with such 

 clearness. 



In at least most other agents in killing concentrations irreversible 

 changes begin before the reversible decrease in respiration has proceeded as 

 far as is the case in cyanides. Notwithstanding these differences in action 

 between cyanides and other agents, the susceptibility relations are in gen- 

 eral the same. Moreover, in all cases thus far known, the same suscepti- 

 bility relations appear with extremes of temperature and with lack of 

 oxygen. 



And finally, the limiting surface of living protoplasm, the plasma mem- 

 brane, is itself alive and therefore the seat of a certain amount of metabolic 

 activity, though in all probability the amount is small as compared with the 

 total metabolism of the cell. In fact, the semipermeability of the plasma 

 membrane is associated with this living condition and disappears when the 

 membrane is killed. It is to be expected, therefore, that permeability itself 

 will vary, at least to some extent, with the rate of metabolic activity in the 

 plasma membrane within physiological limits, but beyond these limits, where 

 irreversible changes occcur, this relation undoubtedly ceases to exist. 



Susceptibility, as determined in my experiments, is primarily concerned 

 Avith, and is, at least to some degree, a measure of the physiological condi- 

 tion of this superficial region of the cell, and differences in susceptibility in 

 different regions of a cell or in different cells of an individual must depend 

 upon differences in this condition. The exact method of action of each par- 

 ticular agent is not yet known, but since the susceptibility relations are 

 essentially the same for at least many different agents within a certain range 

 of concentration or intensity, it is evident that all of them, whatever their 

 particular point of attack on the protoplasm, produce the same general 

 effect, i. e., the retardation or cessation of the fundamental activities of life, 

 and that the rapidity with which these changes occcur shows in general a 



