VERTEBRATE PHOTOSTIMULATION 645 



energy, and it is probable that the center first affected is the brain. In 

 the case of light entering the eyes to stimulate the growth of the 

 gonads, the pathway of the stimulus via the optic nerves to the brain 

 has been clearly demonstrated. Further it has been suspected that the 

 hypothalamus must be a region of special importance in this respect, 

 and that some stimulus must pass from the hypothalamus to the pitui- 

 tary gland, which in turn stimulates gonad growth (Donovan and 

 Harris, 1955). 



It is appropriate to introduce at this point the problem of internal 

 reproductive rhythms. Such internal rhythms have been known for a 

 long time and they account, for instance, for the fact that the spring- 

 breeding English minnow may show gonad growth even when the en- 

 vironment is maintained steadily in the winter condition (Bullough, 

 195 1 ) . The well-known refractory period may also be regarded as part 

 of such a rhythm. It is probable that the reproductive cycles of many, 

 if not most, vertebrates are under the dual control of an internal 

 physiological rhythm and an external seasonal rhythm. Normally these 

 two rhythms coincide and reinforce each other, but if they are made to 

 clash experimentally, then it is the seasonal rhythm that ultimately 

 gains the dominance. 



It used to be susgested that the mechanism of the internal annual 

 rhythm, like that of the shorter estrous rhythm, might prove to be 

 based in the anterior pituitary gland. Today the tendency is to search 

 inside the brain for the seat of both these rhythms, and particularly to 

 examine the function of the hypothalamus, which is in such close con- 

 nection with the pituitary stalk. 



Thus it is possible to trace the pathway of the stimulus from the 

 environment to the brain and perhaps to the hypothalamus, to postu- 

 late that the basis of the internal rhythm may be situated in the neigh- 

 borhood of the hypothalamus, and to note that the hypothalamus is 

 closely connected to the pituitary gland. 



THE HYPOTHALAMUS 



The nervous pathways of the brain, and in particular of the hypo- 

 thalamus, thus stand out as regions of great potential interest to any 

 study of vertebrate photostimulation. The hypothalamus is well de- 



