CONTROL OF ANNUAL GONADAL CYCLES 731 



port of Gomori-positive neurosecretory material. Studies of the hypo- 

 thalami of Zonotrichia leucophrys gambelii in our laboratory (Oksche, 

 Farner, and Laws, unpublished) with a variety of photoperiodic 

 patterns indicate that the situation must be considerably more com- 

 plex. This is emphasized also by the annual cycle of hypothalamic 

 neurosecretory activity in the domestic fowl (Legait, 1955; Legait 

 and Legait, 1955). If one bears in mind the numerous other regula- 

 tory functions of the hypothalamo-hypophysial system and the as yet 

 incompletely understood relationship between the Gomori-positive 

 neurosecretory material and the active regulatory principle, it is 

 obvious that we are contending with a complex situation which 

 requires much additional investigation. 



In the discussion above, it has been indicated, both explicitly and 

 implicitly, that the adenohypophysis is an essential element in the 

 response system. The dependence of gonadal function on the adeno- 

 hypophysis is, of course, general among vertebrates. In birds this has 

 been demonstrated by the marked gonadal atrophy which follows 

 hypophysectomy (see, for example. Hill and Parkes, 1934; Nalbandov 

 and Card, 1943; Nalbandov, Meyer, and McShan, 1946; Frantz, 

 1954); by failure of hypophysectomized birds to show a photo- 

 periodic testicular response (Benoit, 1936c, 1937); by the demonstra- 

 tion of gonadotropic hormones in avian adenohypophyses (see, 

 Greeley and Meyer, 1953; Breneman, 1955; and many others); and 

 by demonstration of gonadal development following injection of 

 gonadotropins in hypophysectomized or intact birds (see, for example, 

 Schildmacher, 1938a; Miller, 1949; Riley and Witschi, 1938; 

 Udintsev, 1948; and Vaugien, 1955, 1956a,b for examples among 

 passerine species; see Breneman, 1955, for a recent resume). 



Figure 4 summarizes schematically, largely on the basis of the 

 investigations of Benoit and colleagues, the mechanism of the photo- 

 periodic response as we envision it at present. 



REFRACTORY STATE OF TESTICULAR PHOTOPERIODIC 

 RESPONSE MECHANISM 



It is a common experience among investigators that there is a 

 period following a complete gonadal cycle, either naturally or artifi- 



