SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 55 



3. Retrogression in Class 2. 



The general appearance of females of this class is not materially 

 different from class I. The total length varies between 14.2 and 

 29.6 mm. The form index averages 4.53 and the fin ratio is 1.17. 

 There are generally fewer primordial ova in the ovary than in the 

 previous class and hardly any one of them appear normal. The 

 most important characteristic of this class lies in the fact that the 

 epithelium of the ovarian cavity is relatively inactive. 



The epithelium of the ovarian cavity is no longer surrounded by 

 germ cells, nor are there many at any other place. Even the stroma 

 is very sparingly represented. The ovary looks empty. The outer 

 ovarian epithelium, although intact, shows similar signs of inac- 

 tivity. Blood vessels are the most prominent structures in the 

 ovary in this class of disintegration. (Fig. 26). 



4. Retrogression in Class 5. 



The size limits in this class vary from 18.4-65.0 mm. in total 

 length. , The average form index is 4.43 and the fin ratio is 1.84. 

 From Table I. it will be seen that the form index of this class is 

 much closer to that of the male than to that of the female, and that 

 the fin ratio is decidedly that of a young male. In fact, in some 

 of the specimens in this class the anal fin has advanced considerably 

 in the process of transformation. These facts are evidently sig- 

 nificant and they will be referred to in connection with the differ- 

 entiation of the male sex. 



Conditions are not less significant as regards the ovary. The 

 outer epithelium of the ovary has been either completely resorbed 

 or else is in advanced stages of resorption. Ova of all stages have 

 left nothing but inconspicuous traces of former presence. Nothing 

 but the epithelium of the ovarian cavity and the oviduct has with- 

 stood the destructive process (Fig. 28). The epithelium has 

 shrunk considerably and is by no means of the same appearance in 

 all cases. It looks inactive in some and active in others, although 

 proliferation of germ cells is not apparent. Judging from the 

 appearance of the ovarian remains, it can be said that the renewed 

 activity of the epithelium in later stages of retrogression is not 

 simultaneous with degeneration, but begins afterwards. Remains 



