VERTEBRATE PHOTOSTIMULATION 647 



also in birds about to migrate (Farner, 1955; Koch and de Bont, 

 1952); and to speculate whether in fact this region of the brain may 

 not only be a highly important link in the chain connecting external 

 light changes with internal gonad growth, but also the controlling in- 

 fluence behind the refractory period and the seat of the seasonal 

 migratory urge. 



Difficult as the hypothalamus is to manipulate, and crude as are our 

 present techniques, it seems evident that a close study of this region 

 of the brain will be an essential prerequisite to a full explanation of the 

 mechanism of photostimulation in vertebrates. 



CONCLUSIONS 



1. Evidence has been reviewed to suggest that visible Hght as such 

 does not induce any specific or essential photochemical transformation 

 in any of those regions of the vertebrate system which are involved in 

 the reproductive processes. In fact external stimuli other than light 

 must be commonly involved in the control of vertebrate reproductive 

 cycles. Any external stimuH, appreciated by the appropriate exterocep- 

 tors, must be translated into patterns of nervous energy which are in 

 turn appreciated by the brain, or perhaps more specifically by the 

 hypothalamus. In all cases it is evidently the increasing nervous ac- 

 tivity which initiates and controls the reproductive rhythm. 



2. It seems probable that most vertebrates are also normally subject 

 to an inherent reproductive rhythm of greater or less strength. It does 

 not appear possible that such a rhythm could be sufficiently accurately 

 timed to control an annual cycle, as was pointed out long ago by 

 Baker when he spoke of the necessity for periodically "putting the 

 clock right." It is by reference to changes in the external world that 

 the internal rhythm is adjusted every year. It is probable that this in- 

 ternal rhythm is also based in some such region as the hypothalamus 

 so that it too may be considered as nervous in origin. 



3. From these suggestions it appears that the onset of a new breed- 

 ing season may be ascribed to a swing in the endocrinological balance 

 induced by the long and steady "pressure" of appropriate nervous im- 

 pulses arriving in the hypothalamus from the exteroceptors. The main 



