104 J- M - ESSENBERG. 



periphery of the acinus and form a lining to the thin membranous 

 wall of the acinus. The lining consists of only one layer and is 

 formed by the nuclei or heads of the spermatazoa. The tails 

 project freely into the lumen. With this the acinus has meta- 

 morphosed into the mature spermatophore which during copu- 

 lation reaches the female genitals by the aid of the gonopod. 



V. DISCUSSION. 



Fertility before Sex- Reversal. 



To eliminate the chance of error and prevent misunderstanding, 

 it was decided that only those females which are definitely 

 known to have produced young will be considered in the study 

 of sex-reversal. The records of female Bi6 shows that she gave 

 birth to 53 young on the 4th of May, and to 40 young on June 

 the 2d, 1923. The records of female C3, which are equally 

 complete, show that only one litter of young were born before 

 she began to change in the male direction. If it is remembered 

 that the females of Xiphophorns helleri reach sexual maturity 

 before they are one year of age, and that the fishes were at least 

 three years of age before they came under the writer's observa- 

 tion, it is seen that females Bi6 and C3 have been experiencing 

 the instincts of motherhood not less than two years before they 

 experienced the instincts of fatherhood. 



Fertility after Sex-Reversal. 



From the mating in which Bi6 was the male, eight young 

 were born. C3 became the father of five young, all of which 

 developed to normal, sexually mature fish. It can be stated 

 that the mortality in the progeny of Bi6 was wholly due to the 

 inability to control temperature in the aquaria. The sex ratio 

 of the young obtained compares favorably with sex ratios found 

 by the writer in his study of sex-differentiation (1923). 



The Origin oj Testicular Tissues. 



It has been shown (Essenberg, 1923) that the definitive germ 

 cells of Xiphophorus helleri are not linear descendants of primor- 

 dial germ cells but originate from peritoneal epithelium. The 

 primordial germ cells either entirely disintegrate (females) or 



