PHOTOPERIODISM IN FISHES 665 



light per day might stimulate pituitary activity as effectively as a 

 supraliminal (hence longer) undisrupted photoperiod. Thus the daily 

 duration of effective stimulation might be increased without increasing 

 the total hours of illumination per day. Some such effect, although 

 differently construed, has been reported for birds. The critical experi- 

 ment has not been performed with fishes, but in Gasterosteus a faster 

 tempo of egg maturation was reported by vanden Eeckhoudt ( 1946) 

 with a daily light ration of 20 min out of each hour gradually increased 

 to 40 than with an undisrupted daily photoperiod of 8 hr gradually 

 increased to 16, the total hours of illumination being the same. It 

 remains to be demonstrated for fishes whether in this context "breaking 

 the dark period (scotoperiod)" or "fractionating the photoperiod" is 

 the more appropriate phrase. If thresholds of responsiveness to photic 

 stimuli are concerned, daily duration of effective stimulation would 

 seem to be the critical feature rather than total hours of daily light 

 per se. 



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