SELIG HECHT 161 



equilibrium will be reached, in which the quantity of precursor will 

 be constant. This will happen at any intensity of light no matter 

 from which direction it is approached. 



Constancy in the quantity of precursor means no stimulation of 

 Ciona. We have thus a mechanism which possesses the first require- 

 ment for an explanation of the photosensitivity of Ciona. This, it 

 will be recalled, is the ability of the animal to come into sensory 

 equilibrium at any intensity of illumination. An explanation of the 

 second characteristic of the sensitivity follows from this at once. 

 In order to act as a stimulus the increase in illumination must be 

 maintained for such a time as to produce a definite fraction of the pre- 

 cursor present. This is constant; therefore, the reaction time will be 

 constant. The third requirement is also easily deduced. The quan- 

 tity of precursor required for stimulation is constant. Since the re- 

 action P-<— 5 follows the reciprocity law, it will make no difference 

 whether the intensity is low and the sensitization period long or 

 vice versa. The variation in the reaction time during dark adapta- 

 tion has already been dealt with to show that it depends on the sug- 

 gested photochemical system. The latter therefore completely ful- 

 fills our demands for an explanation of the photic responses of Ciona. 

 4. It should be pointed out that the proposed chemical system is 

 the simplest, but not the only hypothesis capable of explaining the 

 sensory phenomena observed in Ciona. There are two reactions re- 

 quired: one, a "light" reaction, and the other, a "dark" reaction. 

 It has been assumed that these are the two phases of a reversible 

 reaction system. A somewhat more complex, yet essentially similar 

 hypothesis is to consider the two reactions as forming a catenary 

 system. 



According to this alternative suggestion, a photosensitive sub- 

 stance 5 is decomposed, not into its precursor but into another sub- 

 stance DS. This is the "Hght" reaction. The decomposition product 

 DS then becomes converted into an inert material M. This is the 

 "dark" reaction. Thus 



Ught ^^' ^"""^ "dark" ^'^' 



Reaction (1) occurs in the light and follows the Bunsen-Roscoe law. 

 The decomposition product {DS) is the stimulating substance, and in 



