SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 69 



that they are in stages between a completely disintegrated ovary 

 and the origin of the male gonad i.e., the late stage tubule for- 

 mation. In the second place, breeding experiments have shown 

 that there are no sterile individuals unless diseased, which, as I 

 have pointed out, are not very common. There are several cases 

 where a fish has been selected as a normal female and placed in an 

 aquarium with a male for breeding purposes and after several 

 months two well-formed males have been found in the aquarium. 

 Finally the evidence that such retrogressive females may develop 

 into males may be summarized as follows : 



1. The sex-ratios described above, in which the ratio is reversed 

 at maturity from what it was in immature stages, favors very 

 strongly the idea of sex-inversion. 



2. The occurrence of a bifurcated testis in certain males (see 

 p. 60) which is to be explained as a connecting link between female 

 and male. 



3. The irregular contour of the testis and the large and irregular 

 tubules of the presumed arrhenoids (Figs. 40-41) contrast con- 

 spicuously with the normal testis (Fig. 38). 



4. The condition of the anal fin is further supporting evidence. 

 Making ample allowance for all errors, the average fin ratio of the 

 retrogressive female is far into the domain of the male (Table I.). 

 In fact, the general appearance of such an anal fin suggests the 

 male condition in the stage of late tubule formation, whereas there 

 is nothing more than the remains of the epithelium of the ovarian 

 cavity. 



5. The form index and the advanced age of the arrhenoid fish 

 as compared with normal sex-differentiation of the male are addi- 

 tional arguments in favor of sex-inversion in Xiphophorus helleri. 



The opinion of breeders and fish fanciers has been for a number 

 of years in favor of sex-inversion in Xiphophorus helleri and re- 

 lated forms. Their observations have been confined naturally to 

 external features; mainly to form index and anal fin. I have had 

 the opportunity to meet the members of the Chicago Aquarium 

 Club and hear reports on first-hand observations dealing with the 

 problem. Several members of this club have favored me with 

 material of various kinds for which I wish to express my sincere 

 appreciation. 



