MEMBRANE CHANGES DURING STIMULATION 375 



ings, is scarcely applicable to the case of irritable elements 

 in general. But such facts at least show plainly that 

 stimulation is often associated with increased i)ermea- 

 bility or other evidence of temporary structural break- 

 down; and the fact that the conditions under which 

 stimulation occurs in other irritable li\ing systems, and 

 also its most general manifestations such as the bioelectric 

 variations, are of the same kind in the turgor-motor 

 cells of plants and in gland cells as in muscle and nerve 

 points clearly to the existence of some fundamental 

 physico-chemical condition common to all such ])r()mptly 

 reacting irritable systems. If, as the present theory 

 holds, this condition consists in the temporary alteration or 

 breakdown of the film-structure which surrounds and per- 

 vades all protoplasmic systems, the resemblances are intel- 

 ligible; while in any case the differences are to be attributed 

 to special peculiarities of structure and organization. 



The phenomena of luminescence in animals furnish 

 additional evidence that stimulation is associated with 

 the temporary breakdown or remo\al of semi-permeable 

 partitions within the living system. The production of 

 light in irritable luminescent organisms like Xoctiluca 

 may be regarded as an index of stimulation in \er}' much 

 the same sense as the bioelectric currents are such 

 an index; and probably both phenomena are conditioned 

 by structural changes of a similar kind. The investiga- 

 tions of Dubois and Harvey indicate that in many if 

 not all luminescent animals light-production de])ends on 

 the union of the two photogenic components, lucifcrin 

 and luciferase, in the presence of oxygen.' In a lumi- 



^ Cf. Harvey's recent book, The Nature of Atiimal Lighl (Phila- 

 delphia, 1920). 



