SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 57 



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1. Early Stage of Tubule Formation. 



The size limits of the young male vary from 9.3 to 13.5 mm. in 

 total length. The average form index is 5.17 and the fin ratio is 

 1. 1 6, which differs very little, if any, from that of the female in 

 similar stage of development. 



The gonads, like those of the female, are far apart at the begin- 

 ning of sex-differentiation. Gradual approach follows until they 

 meet medially below the air bladder to be separated only by the 

 mesentery. Unlike 'the ovary, the testes remain permanently 

 paired. The mesentery is well supplied with blood vessels, which 

 at this stage supply the gonads also. There is no difficulty what- 

 ever in distinguishing the testis from the indifferent gonad even in 

 early phases of sex-differentiation. It will be remembered that 

 such difficulty does exist in the case of the female. A comparison 

 of Figs, i and 2 will make this clear. Even in much earlier stages 

 (Fig. 3) this similarity exists. It may be that the indifferent stage 

 is an early female stage. 



Although I have not counted the germ cells in various stages of 

 development, it is nevertheless apparent that the number of the 

 primordial germ cells in the young testis is appreciably less than 

 in the young ovary. Germ cell division was not seen and the cells 

 usually remain isolated. The important role in the differentiation 

 of the male is not played by the primordial germ cells, but by the 

 peritoneal cells. The latter proliferate abundantly, and instead of 

 being evenly mingled with the primordial germ cells, cause a segre- 

 gation of the latter at the periphery of the testis, while the peri- 

 toneal cells occupy the center and inner margin (Fig. 30). This 

 aggregation may be known as the sex-cord and, as will be seen in 

 the next stage, is a preparation for tubule formation. In this stage 

 also some of the peritoneal cells are investing the primordial germ 

 cells. Such activity would indicate follicle formation, which is not 

 an adult male structure. 



2. Middle Stage of Tubule Formation. 



The size limits of animals in this stage vary from 12.5 to 18.6 

 mm. in total length. The form index is 4.84 and the fin ratio 



is 1.27. 



The primordial germ cells play no perceptible role in this stage. 



