EFFECTS OF GONAD GRAFTS IN CHICKS 33 



privilege of examining Mr. Willier's material; I am indebted to 

 him for his kindness in this matter. From my own material 

 and that of Mr. Willier there were available the following em- 

 bryos: twelve fembryos from thymus grafts; eight embryos from 

 thjToid grafts; two embryos from thyroid and thymus grafts, 

 i.e., pieces of both organs implanted on the same embryo; nine 

 embryos from spleen grafts; three embrj^os from kidney grafts, 

 and two from liver grafts. It is understood that in all of these 

 cases the grafts grew. 



In spite of the fact that grafts from these organs grow more 

 readily and faster and attain a larger size than grafts from 

 gonads, in no case was any effect produced upon the urino- 

 genital systems of the embryos which had borne the grafts. - 

 All of the embryos thus obtained are normal males or females, 

 as regards their reproductive systems. I do not wish to imply 

 that these grafts may not have produced some effects upon the 

 embryos, but it may be affirmed with certainty that they had 

 no effect upon the sexual differentiation. 



In view of these facts, it may be conclusively stated that the 

 action of the secretions of the gonads upon the primary sexual 

 characters is specific. Only gonad secretions are capable of 

 modifying the sexual characters in experiments of this kind. 



c. Effect of duration of grafts. An examination of the relation 

 between the duration of the graft and the extent of modification 

 of the embryos will give us some idea of the length of time 

 during which the sex-hormones must act in order to produce an 

 effect. The duration of the graft, that is, the length of time 

 which has elapsed between operation and examination, is a rough 

 measure of hormone dosage employed in each case; the longer the 

 duration, the greater the dosage, presumably. Other factors 

 must, however, be considered. Chief among these is the degree 

 of growth of the graft. Although the same procedure was fol- 

 lowed in each operation, and approximately the same amount 

 of tissue implanted, the mode and degree of growth of the grafts 



- In two cases, however, one with spleen graft and one with liver graft, the 

 left testis showed the same black pigmentation as in the case of several gonad 

 grafts. 



