728 REPRODUCTION AND MIGRATION IN BIRDS 



Although there can be little doubt that it is light per se, or rather 

 orange-red light per se, which causes the photoperiodically induced 

 testicular development, there is far less certainty concerning the 

 identity and nature of the receptors involved. As indicated above, the 

 band of effective wavelengths indicates that the receptors probably 

 can be no more than part of those involved in vision and that some 

 nonocular receptors may be involved. It is of interest to note here 

 that in the domestic duck the pupillary reflex has a maximum response 

 in the yellow, whereas the photoperiodic testicular response has a 

 maximum sensitivity to red-orange (Benoit, Assenmacher, and Walter, 

 1952). It is evident from the experiments of Ringoen and Kirschbaum 

 (1937, 1939) with Passer domesticus and those of Benoit (1934) 

 with domestic ducks that the receptors must be in the ocular region. 

 Benoit and his colleagues subsequently attacked the problem of 

 photoreception in an extensive series of experiments which show 

 quite unequivocally that both ocular and extra-ocular reception are 

 involved. That ocular receptors are involved is clearly evident from 

 comparison of responses of birds with sectioned optic nerves and 

 intact birds to a weak source of light (Benoit and Assenmacher, 

 1953b; Benoit, Assenmacher, and Walter, 1953). Beginning in 1935 

 (Benoit, 1935e,f,h), by means of extraordinarily ingenious and 

 interesting experiments with domestic ducks, Benoit and his colleagues 

 have demonstrated photoreception in the hypothalamus and rhinen- 

 cephalon. (See Benoit 1936a, 1937, 1950b; Benoit and Ott, 1944; 

 and Benoit and Assenmacher, 1953a, for general reviews and descrip- 

 tions of these experiments.) The encephalic sites of photoreception 

 have been detected by conducting light with fine quartz rods and glass 

 tubes (Benoit, Walter, and Assenmacher, 1950a, b). Evidence to 

 support the natural feasibility of encephalic reception was obtained 

 by demonstrating that appreciable quantities of light penetrate to the 

 brain via the orbit of the intact bird (Benoit, Assenmacher, and 

 Manuel, 1952, 1953; Benoit, Tauc, and Assenmacher, 1954a,b). 

 Of particular significance in these experiments is the much greater 

 penetration of red light as compared with green and blue, although 

 the latter are stimulatory if they actually reach the surface of the 

 hypothalamus (Benoit, Walter, and Assenmacher, 1950a). These 

 data have fundamental significance in view of the functional relation- 



