PHYSIOLOGICAL GRADIENTS. 163 



and independent of chemical activity, the axial gradients are 

 manifestly something more than mere permeability gradients. 

 But the protoplasmic limiting surface or membrane is certainly 

 polyphasic in constitution, since it is alive and the seat of more 

 or less chemical activity, and its permeability depends upon its 

 living condition and changes when it dies. Moreover, suscep- 

 tibility as determined by the higher concentrations and intensi- 

 ties of external agents depends rather upon the destruction or 

 alteration of the limiting surface as a living membrane than upon 

 the passage of the agent through the living membrane into the 

 interior of the cell. The susceptibility gradients can be demon- 

 strated not only by agents to which the protoplasmic surface is 

 highly permeable, e.g., vital dyes, various anesthetics, but by 

 those to which it is highly impermeable such as mercury and 

 copper salts, and by extremes of temperature and the negative 

 condition, lack of oxygen, which do not involve the action of any 

 external chemical agent upon the surface. 



A brief consideration of the question of permeability in rela- 

 tion to the action of KNC on Planaria is of interest here. The 

 susceptibility gradients appear very clearly in KNC : in m/iooo 

 KNC, for example, the survival time of the head of Planaria 

 dorotocephala is about half or two thirds that of the least suscep- 

 tible regions of the body-wall. It has been shown, however, that 

 KNC produces a completely reversible decrease of 80-90 per 

 cent, in the oxygen consumption of Planaria (Allen, 'iQa; 

 Hyman, '19)., a fact which certainly indicates that the perme- 

 ability of Planaria protoplasm to cyanide is relatively high, and 

 that the differences in permeability in different regions cannot be 

 very great. If the differences in survival time of different body- 

 regions result from differences in rate of penetration of the 

 cyanide, then the rate of penetration in the most susceptible 

 regions must be at least nearly double that in the least suscep- 

 tible regions, and if this is true, we should expect that in the de- 

 terminations of oxygen consumption some parts of the body 

 would be dead long before a total decrease of 80-90 per cent, had 

 occurred. In fact, it is apparently not possible to interpret sus- 

 ceptibility to cyanide in Planaria solely in terms of rate of pene- 

 tration. 



