160 PHOTIC SENSITIVITY OF CIONA INTESTINALIS 



lus in the sense organ, the quantity of precursor formed by the stimu- 

 lating light bears a definite ratio to the amount already present. As 

 a matter of fact the curve in Fig. 3 is not a theoretical one, but was 

 actually constructed from the data of Fig. 2, on the assumption that 

 the ratio between the precursor formed and the precursor already 

 present is as 1 : 10. The fact that it resembles the ordinary velocity 

 curves for chemical reactions strengthens the force of the above 

 explanation. 



The necessity for the acquisition of a constant ratio between pre- 

 cursor formed and precursor present, before stimulation can occur, is 

 essentially the requirement of the Weber-Fechner concept. This 

 states that the amount of stimulus necessary to produce a perceptible 

 increase in sensory effect represents a constant fraction of the quan- 

 tity of stimulus that has previously been applied. Waller (1895) has 

 already shown that the Weber-Fechner law is dependent upon the 

 processes that go on in the sense organ and not in the transmitting 

 mechanism or in the central nervous system. 



3. It is necessary now to show how the photochemical system sug- 

 gested above can account for the sensory phenomena in Ciona. The 

 investigations of such reversible reactions as proposed here, in par- 

 ticular the work of Luther and Weigert (1905) on the polymerization 

 of anthracene by ultra-violet light, give us some idea of the processes 

 occurring in our reaction system: 



"dark" 

 P "^ S 



light 



The stable condition is that of the sensitive material 5 in the dark. 

 Starting with the precursor P in the dark, it changes spontaneously 

 to S. Light converts 5 back into P. However, because the re- 

 verse change, P~^S, is independent of light, it will be resumed as 

 soon as some P has been formed by the light. Moreover, the more 

 precursor (P) that is formed by the light, the greater will be the rate 

 at which the photosensitive substance will be formed. This follows 

 from the ordinary mass action principle. At a given intensity of 

 illumination the two reactions will proceed in opposite directions at 

 an equal rate, and a ''stationary condition" simulating a chemical 



