THE GONADOTROPIC HORMONES 



secretions on behavior or on secondary sexual characters. 

 The inference that these changes represent the effects of a 

 reflex release of anterior pituitary gonadotropic hormones ap- 

 pears to be justified, because (i) the pars glandularis is as 

 necessary for the maintenance of the avian gonads as for 

 the mammalian, (2) the administration of pituitary gonado- 

 tropic hormone may markedly stimulate the immature or 

 resting gonads of birds or mammals, (3) in some experiments 

 (in the drake, Benoit, 1936; in the ferret, Bissonnette, 1935) 

 it has been shown that the removal of the pars glandularis 

 prevents gonad-stimulation, otherwise occurring as a result of 

 increased illumination, and (4) the anterior pituitary con- 

 tains nerve fibers, probably secretory in character, which 

 have a likely origin in hypothalamic nuclei. To Rowan 

 (1915) belongs the credit for making the first experimental 

 observations in birds. He concluded that prolonged artifi- 

 cial illumination of the junco {Junco hy emails) leads to pre- 

 cocious testicular development during the period of sexual 

 inactivity. Recent studies bearing on the effect of light or 

 the mechanism of this effect have been made in game birds" 

 and in the sparrow and duck. 



In the English sparrow {Passer domesticus) the "ration of 

 light" may largely determine testicular growth so that in 

 midwinter increased illumination may cause precocious testic- 

 ular hypertrophy with spermatogenesis and blackening of 

 the beak. Temperature, as Rowan concluded from his study 

 in the junco, is of little importance. In the spring, however, 

 reduced illumination may partially but not completely sup- 

 press normal testicular development (Kirschbaum and Rin- 

 goen, 1936). From later studies, the authors (Ringoen and 

 Kirschbaum, 1937) concluded that the testicular response to 

 light depends almost entirely upon a "stimulus through the 



" Observations of Clark, Leonard, and Bump (1937) in the grouse, pheasant, and 

 quail. The authors suggested that regression of the gonads in the presence of an 

 apparently adequate photostimulus is due, for unknown reasons, to a lessened 

 anterior pituitary secretion, inasmuch as gonad-stimulation by injected extract can 

 still be produced. 



[59] 



