THE EFFECT OF LIGHT ON BREEDING HABITS 



19 



ber 21 in the northern hemisphere) until the time of their normal breeding 

 season to increasing amounts of light each day. The days get longer during 

 this period of the year. The increase in the amount of light over the amount 

 of darkness each day is the primary factor, then, in controlling the time 

 of ovulation. If in the fall, when the change is in the other direction — 

 that is, when there is more darkness each day and less light — we artificially 

 increase the illumination and thus imitate the spring conditions, breeding 

 will take place in the fall. 



Let us now continue the analysis of the effect of light and examine the 

 results of some experiments on birds, which also react to light. These 

 experiments show that the light has no direct effect on the gonads but acts 

 by way of the pituitary gland. In the first place, if the pituitary gland is 



RATIO OF 



LIGHT 



HORMONES 

 IN 

 BLOOD 

 STREAM 



Fig. 8. Steps in the stimulation of ovulation. The initial stimulus is a vary- 

 ing ratio of light to darkness. The light entering the eye falls on the retina 

 and stimulates nerve impulses, which reach the brain by way of the optic nerve. In 

 the brain the impulses stimulate the pituitary gland to release hormones into 

 the blood stream. The hormones then travel through the heart and circulation 

 to the ovary. In the ovary they produce changes leading to a breaking of the 

 wall of the follicle, and the egg is released into the body cavity or into the 

 oviduct. 



