COMPLETE SEX-REVERSAL IN THE VIVIPAROUS 

 TELEOST XIPHOPHORUS HELLERI. 



J. M. ESSENBERG. 



ANATOMY LABORATORY, SCHOOL OF MEDICINE, 

 UNIVERSITY OF OKLAHOMA. 



I. INTRODUCTION. 



During the progress of the study on sex-differentiation of 

 Xiphophorus helleri it became apparent that sex was not a fixed 

 factor in this species. It was found (Essenberg, 1923) that 

 50 per cent, of the young females transformed to males before 

 sexual maturity was reached. There were also indications that 

 sex-reversal is not confined to immature fishes but that it may 

 occur in adult specimens which have previously born young. 

 To test the merits of such indications experimental plans were 

 conceived which, together with the results, are considered in the 

 present paper. 



With the most favorable environment which prevailed at the 

 Hull Zoological Laboratory and the abundance of Xiphophorus 

 helleri, it was relatively an easy matter to initiate experiments. 

 A large number of adult females were isolated into separate 

 aquaria for observation. Close watch of these specimens was 

 kept for more than a year but with totally negative results. 



In the fall of 1922, the writer became associated with the 

 Medical School of the University of Missouri, where the work 

 was to be continued. Early in the spring of the following year 

 a shipment of fish was received from the University of Chicago, 

 for which I wish to express my gratitude to Dr. A. W. Bellamy, 

 and another shipment was procured from the Crescent Fish 

 Farm of New Orleans. Fishes of both shipments were mixed 

 before they were isolated into separate aquaria. Strict records 

 of each isolated fish were kept. The records were to show the 

 date of isolation, date of birth of young, and the number of 

 young at each birth. 



At the beginning of July, 1923, one of the adult females which 



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