CONDUCTIVITY AND PERMEABILITY 233 



injury occurs, and the resulting reaction produces HGN. 

 Such illustrations might be multiplied indefinitely. 



Another important question which may be considered 

 in this connection is that of mechanical stimulation. The 

 effects of certain kinds of stimuli can be referred directly 

 to chemical changes which they produce in the proto- 

 plasm, but there are other kinds which appear to operate 

 by physical means only. In the latter category are such 

 stimuli as contact, mechanical shock and gravitation. 

 While their action appears at first sight to be purely 

 mechanical, they are able to produce effects so much 

 like those of chemical stimuli that it appears prob- 

 able that in every case their action must involve 

 chemical changes. 



The chief diflficulty which confronts a theory of 

 mechanical stimulation appears to be this : How can purely 

 physical alterations in the protoplasm give rise to chem- 

 ical changes? It would seem that a satisfactory solution 

 of this problem might serve to bring all kinds of stimu- 

 lation under a common point of view, by showing that a 

 stimulus acts in every case by the production of 

 chemical reactions. 



The writer has observed when one of the larger cells of 

 Griffithsia (Fig. 96) is placed under the microscope (with- 

 out a cover glass) and touched near one end (with a needle 

 or a glass rod or a splinter of wood) a change occurs in 

 the chromatophores directly beneath the spot which is 

 touched. The surfaces of the chromatophores in this 

 region become permeable to the red pigment, which begins 

 to diffuse out into the surrounding protoplasm. This 

 change begins soon after the cell is touched. As the red 

 pigment diffuses through the protoplasm it soon reaches 

 neighboring chromatophores and it may then be seen that 

 their surfaces also become permeable and their pigment 



