THE STRUCTURE OF FREE-MARTIN GONADS 105 



cortex would have to be added, since the male gonad never 

 forms normally any homologue of the ovarian cortex. 



Of considerable importance is the demonstration that a mam- 

 mahan gonad with a primary female determination is capable 

 of a graded series of transformations between an ovary and a 

 testis. The exhibition of such a graded series in the free-martin 

 undoubtedly corresponds to the variability in the time of intro- 

 duction, the intensity, or the length of duration of action of the 

 male sex hormones. That the time of introduction may vary 

 is indicated by the variabihty in the anastomosis of the blood- 

 vessels of the two embryos (Lillie, '17). According to this 

 explanation, the gonads most completely transformed in the 

 male direction must have had the most favorable conditions; 

 the sex hormones - were introduced early by an early strong 

 anastomosis of the blood-vessels, or were unusually potent, or 

 the duration of their action may have been prolonged. Obvi- 

 ously, if the conditions are less favorable, lower degrees of 

 transformations will occur. It follows, therefore, that the 

 gonads exhibiting a low degree of transformation must have had 

 the least favorable conditions. To produce such gonads, the 

 sex hormones must have had a late introduction, or were of low 

 potency, or the duration of their action was shortened. 



Such a graded series of transformations is shown by the rela- 

 tive sizes, shapes, and degrees of differentiation of the chief 

 regions of the gonads and the associated epididymis. It has 

 been shoAvn in this paper that the sex-cord region exhibits defi- 

 nite and progressive steps in the male direction. The sex-cord 

 region in the least transformed gonads is small and poorly organ- 

 ized, while in the most highly transformed gonads it is compara- 

 tively large, well organized, and differentiated. The sexual 

 cords themselves exhibit a graded series of transformations 

 between medullary cords and seminiferous tubules. The most 

 highly transformed sexual cords are typical seminiferous tubules 

 in every respect except that the transformation in the male 

 direction has not proceeded to the production of any stage of 

 male sex cells. The only exception to this rule is the brief and 

 unconvincing statement of D. Berry Hart ('10), that in only one 



