644 Growth of Deer Antlers 



ungulates in general, initial stimulation by light, followed by its subsequent 

 withdrawal, may control the hypophysis and determine the sequence of the 

 reproductive events. The present observations on deer draw attention merely 

 to the fact that the annual hypophysial cycle has its onset much earlier than 

 has been suspected from the sexual behavior and that, as in other mammals, 

 light, acting in spring or summer, probably serves as the initial exteroceptive 

 stimulus. It is suggested that similar conditions may prevail in other ungulates 

 which exhibit sexual behavior in the fall. 



In conclusion, a few remarks seem appropriate regarding the question of 

 secondary sex characters. We are accustomed to thinking of the anterior 

 lobe of the hypophysis as acting upon the gonads, but as having no direct 

 influence, in the adult animal at least, upon the Wolffian- or Miillerian-duct 

 derivatives. These latter it is believed are activated by androgens and estrogens, 

 and not directly by the anterior pituitary. Similarly it is generally assumed 

 that the secondary sex characters are also under direct control of the gonads. 

 Thus Riddle" states: "The secondary sex characters (in vertebrates at least) 

 represent or reflect specific responses of developing parts, areas or tissues to 

 a specific hormone of the gonad." Allen*" remarks: "Perhaps the time in 

 evolution when the gonads assumed control of the secondary sex characters 

 will never be known." Whereas Lillie" believes that "all sex primordia or 

 rudiments and, possibly, all sex tendencies, assumed to be common to the 

 life history of both male and female individuals of a species, may be divided 

 in two classes, those that respond positively to the female hormone, and those 

 responsive to the male hormone, each class being uninfluenced by the opposite 

 hormone." 



Nevertheless, in the case of certain secondary sex characters, for example, 

 the antlers, it seems likely that the initiation of their giowth in the adult male 

 deer is under the control of the hypophysis. The subsequent maturation and 

 involution of the antlers, however, appear to be subject to additional regula- 

 tion exerted by the testes. Hence, it seems probable that this particular sec- 

 ondary sex character is under the joint control of both anterior hypophysis 

 and gonads in the manner outlined, with the pituitary providing the initial 

 stimulus. In lower vertebrates most secondary expressions of sex appear to 

 be under complete control of the gonads, yet occasionally in birds, as exem- 

 plified in the plumage of one of the African weaver finches (Witschi"), the 

 pituitary evidently plays a determining role. In mammals, in the case of the 

 mammary glands (Turner^), the hypophysis may also exert a regulating in- 

 fluence. One interesting difference exists between the case of the African 

 weaver finch and the control of antler growth. Witschi's observations indi- 

 cate that both gonads and plumage are controlled by gonadotrophic hormones. 

 In the case of the Virginia deer, in which antler growth precedes gonadal 

 growth by some six weeks to two months, it seems likely that two separate 

 pituitary factors are operative, the first a form of growth hormone acting on 



