102 



SCIENCE 



[N. S. Vol. XLV. No. 1153 



direct proportionality has been identified 

 with Weber's law by Loeb, who says: 



Since this law underlies many phenomena of 

 stimulation it appears possible that changes in the 

 concentration of antagonistic ions or salts are the 

 means by which these stimulations may be brought 

 about. 



In view of the importance of these rela- 

 tions it seems desirable to ascertain what 

 mechanism makes one proportion better 

 than others and preserves this preeminence 

 in spite of changes in concentration. 



Precisely this kind of mechanism is in- 

 volved in the theory just outlined. It is 

 easy to see that such a mechanism must 

 exist if the formation of NaaXCaCl^ takes 

 place at a surface. In a surface substances 

 usually exist in a different concentration 

 from that which they have elsewhere in the 

 solution. If NaCl and CaCla migrate into 

 the surface, so as to become more concen- 

 trated there than in the rest of the solution, 

 their concentration in the surface must in- 

 crease, as their concentration in the solu- 

 tion increases, up to the point where the 

 surface is saturated. Beyond this point an 

 increase in their concentration in the solu- 

 tion produces no effect on their concen- 

 tration in the surface. 



When this stage has been reached the 

 formation of Na,XCaClj, if it takes place 

 in the surface, will not be affected by an 

 increase in the concentration of the salts 

 in the solution. It will, however, be af- 

 fected by changes in the relative propor- 

 tions of the salts. The number of molecules 

 in a unit of surface will remain nearly 

 constant, but if the proportion of NaCl in 

 the solution be increased some of the CaCl^ 

 in the surface will be displaced by NaCl. 



Below the saturation point the relative 

 proportions of the salts will be of less im- 

 portance than their total concentration: 

 this is the case at low concentrations in the 

 region of the so-called "nutritive effects." 



It is evident that if we adopt' this theory 



we can see why the most favorable propor- 

 tion must remain approximately the same 

 in spite of variations in concentration, and 

 we thus arrive at a satisfactory explana- 

 tion of Weber's law. 



There are other ways in which perme- 

 ability appears to be connected with stimu- 

 lation. One of these has to do with an- 

 esthesia. Typical anesthetics decrease per- 

 meability. This accords with the idea that 

 stimulation depends on the movement of 

 ions in the tissue. Such movement would 

 be cheeked by decrease of permeability. 



Another has to do with mechanical stimu- 

 lation. It is well known that the effects of 

 certain kinds of stimuli can be referred 

 directly to chemical changes which they 

 produce in the protoplasm, 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 mechan- 

 ical, they are able to produce effects so 

 much like those of chemical stimuli that it 

 appears probable that in every case their 

 action must involve chemical changes. 



The chief difficulty which confronts a 

 theory of mechanical stimulation appears 

 to be this : How can purely physical altera- 

 tions in the protoplasm give rise to chemical 

 changes ? It would seem that a satisfactory 

 solution of this problem might serve to 

 bring all kinds of stimulation under a com- 

 mon point of view, by showing that a stim> 

 ulus acts in every case by the production 

 of chemical reactions. 



An answer to this question is suggested 

 hy some observations on the cells of the 

 marine alga Griffiihsia. When one of the 

 larger cells is placed under the microscope 

 and touched near one end a change occurs 

 in the chromatophores directly beneath the 

 spot which is touched. The surfaces of the 

 chromatophores in this region become per- 



