348 



SCIENCE 



[N. S. Vol. LIII. No. 1372 



responds only after a measurable interval, 

 whicli is usually longer than one and a half 

 seconds. This interval is called the reaction 

 time. (2) The animals will respond to light 

 only when there has been a decided increase 

 in its intensity. (3) Once a response lias been 

 secured to a given illumination, the con- 

 tinued application of the same intensity fails 

 to produce any additional effect. (4) If, 

 following this, the animal is placed in the 

 dark, it very soon recovers its sensitivity to 

 the light which had previously become in- 

 effective. 



It is apparent that these four characteris- 

 tics are not confined merely to these two 

 species of animals. They belong generally to 

 all animals which are sensitive to increased 

 illumination. Their analysis is therefore of 

 more than immediate interest. Their pres- 

 ence and their quantitative aspects have 

 determined the nature of the hypothesis 

 proposed, and they in turn find their explana- 

 tion in terms of the hypothesis. It will 

 therefore be well to consider these four out- 

 standing characteristics in greater detail. 



The reaction time is the interval from the 

 beginning of the exposure to the beginning of 

 the response. In Ciona this may vary from 

 2 to 10 seconds, and in Mya from 1.5 to 4 

 seconds, depending on circumstances such as 

 temperature, intensity of light, and duration 

 of exposure. If these are kept constant, the 

 reaction time is constant. 



Fortunately this reaction time is made 

 up almost entirely of the time lost in the 

 sense organ. For example, mechanical stim- 

 ulation produces the same reflex as illumina- 

 tion. Yet the reti-action of the siphons occurs 

 so rapidly that it is not possible to measure 

 it with a stop watch. The adjustor and 

 effector processes, therefore, take almost no 

 measurable time, and the reaction time is 

 confined to the processes which take place 

 in the receptor. This is, to say the least, 

 highly convenient. 



The reaction time, however, is not a simple 

 interval. The total exposure to light, which 



it represents, is not necessary. If the animal 

 is exposed for, say, half the reaction time, it 

 will still respond in the darlc at the end of 

 the usual reaction time. By proper methods 

 it is possible to reduce the exposure and at 

 the same time to measure the reaction, time. 

 It is found that for each intensity of light 

 there is a minimum exposure which will cause 

 a response at the end of the usual reaction 

 time. This short exposure is the sensitization 

 period. Exposures longer than the sensitiza- 

 tion period make no change in the duration 

 of the reaction time; exposures shorter than 

 the sensitization period prolong the reaction 

 time, as will presently be described. That 

 portion of the reaction time during which the 

 animal is in the dark, or during which the 

 exposure to light is not necessary, is called 

 the latent period. S"ormally, therefore, the 

 reaction time is composed of two parts: a 

 sensitization period and a latent period. 



The whole matter is strikingly illustrated 

 with Mya. Here the sensitization period is 

 extremely short. With a strong light it is 

 only a few hundredths of a second long, 

 whereas the latent period comprises most of 

 the reaction time, which in such a ease is 

 about one and a half seconds. 



The sensitization period varies with the in- 

 tensity. The latent period however, provided 

 certain conditions are maintained, is con- 

 stant for all intensities. At room tempera- 

 tures the latent period for Ciona is 1.76 

 seconds; for Mya it is 1.31 seconds. Since 

 it is our purpose to study the quantitative 

 aspects of this photic sensitivity, it is ap- 

 parent that the analysis of the reaction time 

 into its two constituents is of first rate 

 significance. The composition of the reaction 

 time was first discovered with Ciona, and it 

 immediately opened a hitherto inaccessible 

 field of investigation. 



The second characteristic of the sensitivity 

 of these animals is the fact that they will 

 respond to light only when it is increased. 

 This initial action of the light must be on a 

 photosensitive substance contained in the 



