MEMBRANE CHANGES DURING STIMULATION 359 



effects receive a consistent explanation on the theory of 

 variations of permeability. 



The implication that apparently inert cells like 

 epidermal cells are irritable, in the same sense as muscle 

 and nerve, may seem a strange one, but it is in harmony 

 with the general conception of irritability as an ele- 

 mentary property of all forms of living matter. Waller 

 has pointed out that the most certain ''sign of life" in an 

 apparently inert animal or plant tissue is the elicitation 

 of an electric response (''blaze-current") on mechanical 

 stimulation.^ The increased proliferative activity of 

 the epidermal cells of the skin after hard mechanical 

 usage is well known. Ebbecke's experiments show that 

 such treatment increases the electrical conductivity; 

 in other words, increases permeability and decreases 

 polarizability, as in other cases of stimulation. The 

 increased growth is the expression of this stimulation. 



Recent observations by Crozier^ have also a bearing 

 on the present problem. He finds that electrical 

 stimulation causes- a well-marked increase in the permea- 

 bility of the mantle-cells of nudibranchs to acids; in 

 these cells the degree of permeability can be measured 

 by the time required to change the color of an intracellular 

 pigment, which acts as a natural indicator. Mg salts 

 and anaesthetics (in appropriate concentrations) were 

 found to decrease permeability, as in the cases already 

 cited.^ Mechanical traction causes an increase of 

 permeabihty, which within certain limits is reversible. 

 These observations throw much light on the general 



* Waller, Signs of Life. 



^ Jour. Gen. Physiol. ^ IV (1922), 723. 



3 Cf. chap. viii. 



