20 EMBRYOLOGY 



removed the increased light has no effect on the gonads. Another bit of 

 evidence indicating that the pituitary is the stimulating factor is that injec- 

 tions of pituitary will produce the same effect as light. Therefore light must 

 act by a release of the gonadotropic hormone. Experiments in which animals 

 are hooded — so that their eyes are in darkness — show that there is no effect 

 of light unless the light reaches the eye. That is, any absorption of light on 

 the body surface, or any effect of light through heat, has no effect. It must 

 reach the eye. The light must, furthermore, give rise to nerve impulses, be- 

 cause if the optic nerve is cut and the animal is exposed to light, no effect 

 is obtained. 



Thus we have traced the stimulus for ovulation back to a periodicity of 

 light falling on the retina, where it is converted into nerve impulses which 

 reach the brain through the optic nerve. In the brain the nerve stimulus 

 passes to the pituitary gland and stimulates the release of gonadotropic hor- 

 mones. These travel through the blood stream and when they reach the 

 ovary they cause the breaking of the wall of the follicle and the release of 

 the egg (Fig. 8). 



It should be pointed out here that this treatment with light is not effec- 

 tive for all animals, but it has been successfully used in several fur-bearing 

 animals which have seasonal cycles and in several species of birds. On the 

 other hand, such animals as the guinea pig, rat, and man have some internal 

 control which is independent of light and to some extent of other environ- 

 mental conditions. 



Internal control of pituitary secretion 



What is the control of ovulation in animals which breed without relation 

 to the environment? In these forms, too, the pituitary controls ovulation, but 

 hormone secretion by the pituitary is controlled by a rhythmic factor which 

 we shall attempt to analyze. 



Let us first outline the changes that occur in the ovary (Fig. 9). The egg 

 matures in a follicle, which increases in size until its wall breaks and the egg 

 is liberated into the opening of the oviduct. The follicle shrinks after ovula- 

 tion and becomes filled with cells; these form a solid body, the corpus luteum. 

 The corpus luteum then increases in size for a while but finally degenerates, 

 and a new follicle begins to enlarge in some other region of the ovary. This 



