SELIG HECHT 665 



IV. 



In terms of the conception developed in the present paper, the 

 latent period assumes a position of importance in the composition 

 of the reaction time of My a to light, coordinate with that of the 

 sensitization or exposure period. There being no evidence to the 

 contrary, it seems reasonable to assume that the processes under- 

 lying these two phases of the reaction time occur entirely within the 

 sense organs. The events which happen in the sense organ of Mya 

 when it is stimulated by light may, according to our findings, be ex- 

 pressed as follows. The photosensitive substance (5), originally 

 formed from its two precursors (P and A, — Precursor and Acces- 

 sory), is changed back into them under the influence of light, both 

 reactions being given by the expression 



light 

 S ^ P-\-A. 

 "dark" 



This happens during the exposure to light or during the sensitiza- 

 tion period when the exposure is prolonged. One or both of the 

 freshly formed precursor substances then immediately serve to 

 catalyze the transformation of an innocuous material (Z) into a 

 stimulating substance (T). This occurs during the latent period. 

 When a sufficient amount of the stimulating substance {T) has been 

 accumulated, it acts on the nervous connections to the sense organ 

 and initiates the retraction of the siphon. The entire sensory 

 process may therefore be summed up in the two reactions: 

 S-^P + A; L\\P + A\\-^T 



in which the symbol \\P + A\\ signifies catalysis by one or both of 

 the precursor substances. 



Two reactions related as are these correspond to a well known 

 group of photochemical reactions classed by Weigert as "catalytic 

 photochemical reactions in which the catalyst remains after the 

 action of light." Among these are included so familiar a process as 

 the exposure of a photographic plate and the development of the 

 image under proper conditions (Weigert. 1911, p. 74). 



The hypothesis of photoreception, here presented rather con- 

 cretely, will undoubtedly have to be modified in one way or another 



