COMPLETE SEX-REVERSAL IN VIVIPAROUS TELEOST. 1 03 



peritoneum undergo characteristic transformation. The cyto- 

 plasm as well as the nucleus increases in size and assumes a 

 spherical form. When these cells have completed their differ- 

 entiation they can not be distinguished microscopically from 

 the primordial germ cells of the indifferent stage. 



IV. THE THIRD STAGE IN SEX-REVERSAL. 



The third stage in sex-reversal is characterized externally by 

 the development of a "sword "-like process in the tail fin and 

 internally by spermatogenesis. 



It is true that there is a slight overlapping between the second 

 and third stages, yet it is desirable to consider these separately 

 for the reason that they constitute definite landmarks in the 

 process of sex- reversal. 



The sword of the caudal fin begins to develop slightly before 

 the gonopod reaches its completion. It is formed from the 

 ventral lobe of the tail fin. The ten ventral rays of this fin are 

 involved. Most of the elongation falls upon the sixth to the 

 tenth and the utmost length is reached by the eighth ray. The 

 length of the fully formed sword approximates the total length 

 of the fish. It is abundantly supplied with brilliant colors and 

 its sole purpose seems to be display. 



The development of the testis in this stage of sex-reversal 

 consists of spermatogenesis. A number of germ cells develop 

 in a constricted portion of the sex-cord or tubule and form an 

 acinus or spermatocyst. Every cell in such a cyst is in the 

 same stage of development from the beginning to the end and 

 they ultimately form the spermatophore. At the end of the 

 so-called "Middle Stage of Tubular Formation," the germ cells 

 are primary spermatocytes. The secondary spermatocyte stage 

 is reached by cell division which doubles the number of germ 

 cells and the volume of the acinus. With another cell division 

 which doubles the cell number but not volume of the acinus 

 the germ cells reach the spermatid stage. With the meta- 

 morphosis of spermatids into spermatozoa the acinus which thus 

 far has been so tightly packed with germ cells that it simulates a 

 syncytium now becomes luminated. The lumination owes its 

 origin to the fact that the spermatozoa move towards the 



