EndodHne Mechanisms 751 



shown that the testis of the male normally reaches a peak of its annual cycli- 

 cal activity in the late spring. Experiments on male juncos, sparrows, and 

 starlings have clearly shown that gradual increase in length of the daily pe- 

 riod of illumination through supplementary artificial lighting will bring the 

 activity of the testes to a maximum at a time not typical for the species 

 Properly controlled experiments have shown that it is the light itself rather 

 than the resulting longer daily periods of activity or feeding which is the 

 actual determining factor. A similar subjection to increased periods of il- 

 lumination over a period of about two months has been found to bring fe- 

 male ferrets into estrus in winter, a time at which in their normal reproduc- 

 tive cycle they are in anestrus.-^ Similar observations have been made on 

 mice CPeromyscus^.^^''' Among the fishes the reproductive cycle of the trout 

 has also been shown to be importantly influenced by photoperiodism."^^ 



In the field mouse, Microtus, the gonads have been shown to diminish in 

 size during treatment of the animal with gradually decreasing periods of il- 

 lumination.^^ 



Such experiments as these appear to indicate that the gradual changes in 

 day-length in the annual solar cycle constitute a very effective stimulus in 

 determining the normal annual reproductive cycles of many species of verte- 

 brates. 



In the immature duck whose gonads are normally stimulated by increased 

 illumination, it has been shown that pituitaries from illuminated specimens 

 show increased gonadotropic activity over those of untreated controls when 

 implanted into immature mice. Red and orange lights are more effective 

 than other colors in inducing the effect. The eyes are not essential to the 

 reaction; direct illumination of the pituitary resulting from illumination of 

 the orbit after enucleation effectively activates the gland. When light is con- 

 ducted directly to the pituitary through a quartz rod, blue light, otherwise 

 showing only small influence, becomes quite as effective as red and orange. 

 Similar action of light on the reproductive cycle through other routes than 

 the eyes has been shown for sparrows. In the ferret, on the other hand, di- 

 vision of the optic nerves completely inhibits the action of light, indicating 

 the retina of the eyes to be the effective receptor agency involved in this 

 species. 



Nervous connection of the hypothalamus with other parts of the nervous 

 system is usually essential to normal sexual functioning. Nervous con- 

 nection between the hypothalamus and the pituitary is essential for certain 

 aspects of control of the reproductive cycle, as for example ovulation in the 

 rabbit, but appears non-essential in numerous other instances, the conduction 

 in these latter cases apparently being humoral. 



The tem.perature of the external environment has been shown to play 

 an important role in determining the time of gonadal development and ap- 

 pearance of secondary sex characters in the stickleback (Gasterosteus), a 

 sufficient rise in temperature rapidly inducing these changes. These fish 

 show no significant responses to increasing illumination. Low temperature 

 and darkness such as normally obtain during its annual period of hiberna- 

 tion will activate the gonads in the ground squirrel, Citelliis, at any season. 



The general state of nutrition of an organism, as well as the sufficiency 

 of specific vitamins such as A, E, and the B complex, have an important ef- 



