234 INJURY, RECOVERY, AND DEATH 



begins to diffuse out. In this way a wave — which, may be 

 compared to a wave of stimulatioai — progresses along the 

 cell until the opposite end is reached. 



The rate of propagation of this wave corresponds to 

 that of the diffusion of the pigment. It would seem that 

 at the point where the cell is touched, pigment, and prob- 

 ably other substances, are set free, diffuse out and set up 

 secondary changes as they progress. These changes are 

 doubtless chemical in nature. 



The important question then arises: How does the 

 contact initiate the outward diffusion of the pigment or 

 other substances? 



It seems to the writer that this may be due to a 

 mechanical rupture of the surface layer of the chromato- 

 phore which is either not repaired at all or only very 

 slowly. Many cases are now known in which the surface 

 layers of protoplasmic structures behave in this way.®^ 

 If, therefore, such structures exist within the cell, it is 

 evident that any deformation of the protoplasm which is 

 sufficient to rupture their surface layers, will permit their 

 contents to diffuse out into the surrounding protoplasm. 

 A great variety of cellular structures (plastids, vacuoles, 

 "microsomes," inclusions, etc.), possess surface layers of 

 great delicacy, and it is easy to see how some of these 

 might be ' ruptured by even the slightest mechani- 

 cal disturbance. 



It would seem, therefore, that deformation may rup- 

 ture the surface layers of certain structures and cause 

 their contents to diffuse out. If the diffusing substances 

 meet others, from which they were separated by the semi- 

 permeable surface layer before it was ruptured, it is 



" In many caaes rupture of the plasma membrane causes the proto- 

 plasm to disintegrate and mix with the surrounding medium. In other 

 cases the surface layer is at once reconstituted. 



