142 



EMIL WITSCHI. 



With regard to the last two points, the above statements 

 are in full agreement with Lillie's observations on free-martins. 

 However, while the latter never effect a complete sex-change, and 

 do not produce male germ cells, the changing frog twins ap- 

 parently are on the way to become true males. In frogs, ex- 



FIG. 4. Parabiotic twins 8840 (R. sylvatica). Right animal shortly after 

 metamorphosis. 68 days after the operation. Female changing into male. 

 Transverse section through the posterior part of the gonad which shows nearly 

 typical male conditions. Highly developed sex cords, relatively small number of 

 spermatogonia. Rests of the former germinal epithelium at mig. (migrating germ 

 cells) and deg. (degenerating oogonia). X 444- 



hibiting a transformation of sex, the undifferentiated germ-cells, 

 which in the ovary are exclusively found in the peripheral 

 germinal epithelium (so-called primary oogonia, Fig. 3, og.} 

 migrate to the center of the gonad (Fig. 5, mig.}. They join 

 the sex-cords, to form the characteristic lobules, which are 

 known to be the anlagen of the seminiferous tubules (Fig. 4), 

 (Witschi, 1914; Swingle, 1926). From this stage they are there- 

 fore to be considered as spermatogonia (Fig. 4, spg.}. 



The described facts are in many respect in apparent contrast 



