SEX-REVERSAL IN PARABIOTIC TWINS. 145 



"hormones" taken in its proper sense, would indicate, that the 

 sex-differentiators circulate in the blood and are spread through- 

 out the entire body. The writer's previous studies on sex- 

 differentiation in the frog seem to point in a somewhat different 

 direction. In hermaphrodites the eggs are formed exclusively in 

 the germinal epithelium, while the spermatogonia develop in the 

 sex-cords. At the beginning of the sex-reversal from female to 

 male those germ-cells situated closest to the sex-cords are the 

 first to show characteristic changes and to migrate into the sex- 

 cords. If we consider the behavior of the germ-cells as an 

 indication of, or a reaction to, the nature of their environment, 

 then we must attribute male differentiating properties to the 

 sex-cords. On the other hand, from the fact that the germ-cells 

 which were not attracted by the sex-cords, but which remained 

 in the germinal epithelium, become eggs, it becomes evident 

 that this epithelium includes female-differentiating properties. 

 From such facts, which furthermore are supported by experi- 

 mental data, I proposed the theory of localized sex-differentiators 

 (" lokalisierte Innenfaktoren," 1914). These probably are com- 

 parable to Spemann's "Organisatoren." 



The fact of independent sex-differentiation in genetically two- 

 sexed twins favors again that localization theory. Regarding 

 the secondary sex-reversal of the female part, the cooperation 

 of hormones is not improbable. We may assume, either that 

 part of the active substance elaborated in the sex-cords is carried 

 away by the blood-stream, or that the differentiated testes 

 produce a hormone, which is different from the substance 

 inducing the first sex-differentiation. Yet, it is to be kept in 

 mind, that at least in the frogs there is no definite proof, 

 that the testes of the male twin act by way of the blood-stream. 

 This question has to be discussed in connection with a detailed 

 study of the homoplastic and heteroplastic twins, which will be 

 published at a later date. 



BIBLIOGRAPHY. 



Bascom, K. 



'23 Interstitial Cells in Cattle. Am. Journ. Anat., Vol. 31. 

 Bissonnette, Th.H. 



'24 The Development of the Reproductive Ducts and Canals in the Free- 

 martin with Comparison of the Normal. Am. Jour. Anat., Vol. 33. 



