1138 



SUBMAMMALIAN VERTEBRATES 



ions experiments Benoit and Ott (1944) 

 investigated the relationship between wave- 

 length and response. They established that 

 red and orange light, which were more ef- 

 fective in intact birds than was blue, pene- 

 trated deeper into the tissues of the brain 

 than the blue. These observations have been 

 confirmed by more refined techniques (Ben- 

 oit and Assenmacher, 1959). Subsequently, 

 a quartz rod was used to shine the light di- 

 rectly on the hypophyseal-hypothalamus 

 area; under these conditions, blue light was 

 more effective than red (Benoit and Ott, 

 1944) . 



Benoit and his co-workers have made a 

 careful analysis of the receptors for the light 

 stimulus. In drakes, sectioning the optic 

 nerve reduced, but did not abolish, the 

 gonadal response to increased light (Benoit 

 and Assenmacher, 1955, 1959) . However, 

 when the drake heads were covered with 

 black cloth no stimulation occurred ex- 

 cept when the eyes were left uncovered, in- 

 dicating that two sets of receptors might 

 exist which allow photostimulation to stim- 

 ulate the pituitary. Further investigations 

 showed that light applied directly on the 

 hypothalamus or the rhinencephalon, by 

 means of a quartz rod was effective in gon- 

 adal stimulation (Benoit and Assenmacher, 

 1959). Therefore, a set of receptors con- 

 nected with the optic nerve and a set of 

 deep receptors may be involved in the stim- 

 ulation of the hypothalamus. In mammals, 

 Knoche (1956, 1957) has demonstrated un- 

 myelinated nerve fibers which originate in 

 the optic chiasma and, coursing through the 

 lamina terminalis, reach the ependyma of 

 the third ventricle as well as the para ven- 

 tricular nucleus and the nucleus tuberis in- 

 fundibularis. These fibers would provide a 

 connection between the retina and hypo- 

 thalamus. Whether or not similar nerve 

 fibers are present in the avian brain is un- 

 known. 



The concept presented by Rowan (1938a, 

 b), that light causes gonadal stimulation by 

 inducing wakefulness, which, in turn, af- 

 fects the physiology of the entire body, 

 seems erroneous in view of the evidence now 

 available. Bissonnette (1930) was unable 

 to induce gonadal development by forced 

 exercise. As a matter of fact, when starlings 



were exposed to light, forced exercise had a 

 slight inhibitory effect, which may, however, 

 not have been statistically significant in 

 view of the variability in testes size and the 

 small number of birds used. Benoit (1935) 

 approached the problem by immobilizing 

 drakes and exposing them to light. No dif- 

 ference in response was obtained between 

 free-roaming and immobilized drakes to in- 

 creased photoperiods. The present concept 

 of the manner in which light induces gon- 

 adal stimulation is that it causes an in- 

 crease in the secretory activity of the neuro- 

 secretory cells; the NSM is subsequently 

 transported down the axons and is picked 

 up by the portal vessels in the special zone 

 of the median eminence and transported to 

 the anterior pituitary where it can have its 

 effect. 



The duration of the jihotoperiod required 

 to obtain a gonadal response has been 

 studied in detail by Marshall (1959) and 

 by Wolf son (1959a, b). The regulation of 

 the gonadal cycles of l)irds in the Northern 

 zone seems to be largely, but not entirely, 

 regulated by the photoperiod. A short 

 summary of the events in the natural 

 breeding cycle will clarify the experimental 

 ai^proach used in studies of regulation of 

 the breeding cycle. In the spring the testes 

 and ovaries mature, and, under good con- 

 ditions, breeding starts. At the end of 

 spring or in early summer, the gonads re- 

 gress. The testes show steatogenesis of the 

 tubules, the tunica albuginea is renewed, 

 and a new generation of Leydig cells is 

 formed. During this period, increases in 

 ])hotoperiod will not cause recrudescence of 

 the testes. Marshall (1959) believed, on the 

 basis of the histologic appearance of the 

 testes, that the lack of response was the 

 result of unresponsiveness of the testes. 

 However, earlier experiments (Riley and 

 Witschi, 1938; Miller, 1949) had demon- 

 strated that the gonads can respond to gon- 

 adotrophin administration. Lofts and Mar- 

 shall (1958) confirmed this response with 

 small doses of gonadotrophins and aban- 

 doned the idea that the refractoriness to 

 light was caused by unresponsiveness of the 

 testes. In any event, the testes will not 

 respond to light for a time interval after 

 regression which depends on the species 



