AVIAN MIGRATION AND REPRODUCTION 691 



active at the beginning of the experiment. These results can be 

 interpreted as supporting the inhibitory role of long periods of dark- 

 ness, and it is tempting to do so. It can also be argued that a 15-min 

 period of darkness did not allow enough time for the complete use of 

 the carry-over periods of the 1-hr photoperiods and, hence, the 

 effective photoperiods and carry-over periods could not exceed a total 

 of about 10 to 12 hr. The data did not permit a clear-cut choice 

 between these alternative explanations. 



If a long dark period was inhibitory, what would happen if such a 

 period was used after a 1L-2D schedule? This was done with the 

 following schedule: ( 1L-2D)'-1L-16D. The birds showed an excellent 

 response. Since an effective period of light treatment was not inhibited 

 by a known inhibitory dark period, these results demonstrated the 

 importance of the light period as the effective stimulus. But this 

 interpretation is also open to question. In this experiment the cycle 

 was 38 hr long. The effective light period, despite interruptions with 

 dark periods, was 22 hr long, whereas the dark period was only 16 

 hr long. In a 38-hr cycle perhaps a dark period of 16 hr is not 

 inhibitory. 



Another experiment employing a cycle longer than 24 hr also 

 appears to support the importance of the light period. The schedule 

 used was 12L-16D, and it was designed to test the simultaneous 

 effect of a daily period of light which was known to be stimulatory 

 (12L) and a daily period of darkness which was known to be in- 

 hibitory (16D). The experiment began in December and continued 

 to May. The gonadal and fat responses were excellent and showed 

 some similarity to these responses under a schedule of 12L-12D. 

 These results, in my opinion, point to the light period as the effective 

 period, but again the experimental design does not permit a clear-cut 

 choice. Further studies employing longer dark periods are needed 

 before a final conclusion can be reached. If such studies show that 

 the daily response to an effective daily photoperiod cannot be negated 

 by a strongly inhibitory dark period, as this experiment suggests, then 

 it is obviously the light period that is the effective part of the photo- 

 periodic cycle. The inhibitory role of the dark period would then be 

 merely a matter of semantics. A long night would be inhibitory only 

 because it did not permit a longer or effective period of light. 



