162 PHOTIC SENSITIVITY OF CIONA INTESTINALIS 



order to act as a stimulus the amount of it to be formed by the Hght 

 must bear a constant ratio to the amount of DS already present in 

 the system. Reaction (2) removes the decomposition product from 

 the reaction system. It possesses the same velocity and properties 

 as the ''dark" phase of the reversible reaction system presented be- 

 fore. Its occurrence is represented by the process of dark adaptation 

 shown in Fig. 2, and its course is given by the curve in Fig. 3 in 

 which P represents DS and 5 represents M. 



Which of the two suggested photochemical systems comes nearer 

 the truth future work must decide. The present data may be ex- 

 plained in terms of either. The simpler one is therefore provisionally 

 adopted. 



VII. Repeated Stimulation. 



1. There are several predictions which may be made on the basis 

 of the hypothetical chemical system which has been proposed. My 

 stay at La Jolla, however, limited me to an investigation of but one of 

 these. 



We have seen that the "dark" reaction (P~^S) is a comparatively 

 slow one, requiring several hours to reach an equilibrium. There- 

 fore, if a thoroughly dark-adapted Ciona is stimulated by light at 

 short intervals, say every minute, practically no new photosensitive 

 material will be regenerated during a period of about 10 minutes. 

 The quantity of precursor however will be continually increased, de- 

 pending upon the duration of the exposure to light. This follows from 

 the Bunsen-Roscoe rule which has been shown to apply to Ciona. 

 The quantity of precursor necessary to stimulate must be a constant 

 fraction of the amount of precursor already present. Therefore, dur- 

 ing such a series of regular stimulations, the amount of precursor re- 

 quired must be larger at each succeeding reaction. The rate of in- 

 crease in the amount of precursor will be logarithmic, each quantity 

 representing the same fraction of the sum of the quantities that have 

 been formed before. The sensitization period measures the quantity 

 of precursor formed at each reaction. Therefore, in such a set of re- 

 peated stimulations, the sensitization periods should always bear a 

 definite ratio to the sum of the sensitization periods that have pre- 

 ceded it. 



