74 J- M- ESSENBERG. 



apart, suspended in a peritoneal sac immediately below the air 

 bladder. 



3. The indifferent gonad consists of two kinds of cells : pri- 

 mordial germ cells and cells of peritoneal origin. Both are evenly 

 distributed in the gonad. 



4. At 10 mm. in length sexes are distinct. In females the in- 

 different gonad develops into an ovary without any marked mor- 

 phological changes of the gonad except that the primordial germ 

 cells gradually enlarge and become oocytes. 



5. In males distinct changes take place in which the germ cells 

 and the peritoneal cells are segregated, the former on the periphery 

 of the gonad, the latter occupy the median and inner portions of 

 the gonad. 



6. The paired gonad in the female fuses to form a single ovary. 

 The median surface of the fusing gonads become the ovarian 

 cavity. 



7. In the male the gonads remain paired permanently. 



8. Both the oviduct and the sperm duct are formed from two 

 sources : posteriorly from the peritoneal lining of the body cavity 

 and anteriorly from the gonad. 



9. All females from about 12.5-26 mm. in total length are sub- 

 ject to retrogression. To all appearances all primordial germ cells 

 disintegrate. Definitive germ cells come from peritoneal cells. 



10. Completely disintegrated ovaries have been found with epi- 

 thelium of the ovarian cavity appearing from very inactive to very 

 active. The anal fin in such cases is in early stages of transfor- 

 mation into a gonopod. 



11. The fate of the primordial germ cells in the male is uncer- 

 tain. Definitive germ cells originate from peritoneal cells. 



12. Bifurcated testes occur in a large number of cases, which 

 are supposed to originate from a completely disintegrated ovary. 



13. Sex-ratios are reversed from immature to mature condition. 



14. Differential viability is in favor of the female i.e.. the 

 female is at least as sturdy as the male. 



15. The material decidedly favors sex-inversion in Xiphophorus 

 hcllcri. This takes place most commonly in immature fishes, but 

 may occur in adult animals. 



16. The transformation of the anal fin into a gonopod takes 



