PHYSIOLOGICAL BASIS OF PATTERN 41 



meability and the gradient must depend very largely 

 upon the terms in which we define permeability. If 

 permeability is dependent only on the physical condi- 

 tion of the limiting surface and independent of chemical 

 activity the axial gradients are manifestly something 

 more than mere permeability gradients. But the pro- 

 toplasmic 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, susceptibility as determined by the 

 higher concentrations and intensities 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 demonstrated 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 con- 

 dition lack of oxygen, which do not involve the action 

 of any external chemical agent upon the surface. 



Moreover, the phenomena of differential acclima- 

 tion and recovery in growth and development indicate 

 very clearly that the metabolic activity of protoplasm 

 is a factor in susceptibility. With the advance of our 

 knowledge it becomes increasingly evident that the 

 factors concerned in the permeability of living proto- 

 plasm are essentially those concerned in other aspects 

 of life and that permeability is an expression of the 

 physiological state of the plasma membranes. If we 



