102 J- M. ESSENBERG. 



such as slight fluctuation in temperature, water composition or 

 disturbance will inevitably result in the death of the fish. It 

 seems probable that sex-reversal in Xiphophorus helleri is not 

 preceded by the increase of metabolic rate but that the opposite 

 is true. Nor is it true that the transformation is caused by 

 tuberculosis or any other kind of disease as far as it can be 

 detected either macro- or microscopically. 



III. SECOND STAGE IN SEX-REVERSAL. 



The second stage in sex-reversal deals with the transformation 

 of the anal fin of the female into a functional intromittent organ 

 or gonopod and with the parallel development of the gonad. 



The anal fin of the female consists of ten pairs of rays all of 

 which are approximately of the same length and diameter. 

 The above statement applies equally well to the anal fin of 

 young fishes during the indifferent stage of development. During 

 sex-differentiation in the male direction or sex-reversal, which in 

 Xiphophorus helleri is always from female to male, several pairs 

 of the rays undergo a peculiar metamorphosis to form a copu- 

 latory organ. The first sign of such transformation is the 

 thickening of the third pair of rays. While this is in progress, 

 the third, fourth, and fifth pairs of rays elongate until approxi- 

 mately twice the length of the original fin is reached. The 

 first, second, and sixth to tenth pairs of rays are subject to no 

 special changes and remain rudimentary. The third pair reaches 

 a thickness several times its original diameter and serves as a 

 supporting bar or rod of the gonopod. The apex of this pair of 

 rays together with the fourth and fifth pairs form hooks and 

 counterhooks which serve the purpose of anchorage to the 

 female genital pore during copulation. It is not a hollow penis- 

 like structure but a solid bar which acts as a guide in transmitting 

 of spermatophores from male to female genitals. 



The development of the gonad during the second stage of sex- 

 reversal corresponds to the " Late Stage of Tubular Formation " 

 in sex-differentiation. The solid sex-cords which were formed 

 during the first stage acquire a lumen and become branched 

 and subbranched. The cells constituting the sex-cords or 

 tubules, which thus far were not unlike their sister cells of the 



