SEX-DIFFERENTIATION IN THE VIVIPAROUS 



TELEOST XIPHOPHORUS HELLERI 



HECKEL. 



J. M. ESSENBERG, 



HULL ZOOLOGICAL LABORATORY, 

 THE UNIVERSITYY OF CHICAGO. 



CONTENTS. 



I. Introduction 46 



II. Material and Methods 47 



III. The Indifferent Stage 48 



IV. Early Sex-differentiation of Females 49 



1. Early Normal Ovary 50 



2. Retrogression in Class i 53 



3. Retrogression in Class 2 55 



4. Retrogression in Class 3 55 



V. Early Sex-differentiation of Males 56 



1 . Early Stage of Tubule Formation 57 



2. Middle Stage of Tubule Formation 57 



3. Late Stage of Tubule Formation 58 



VI. Late Sex-differentiation of Females 6l 



VII. Late Sex-differentiation of Males 62 



VIII. Sex-ratios 66 



IX. Inversion of Females 68 



X. Comparative 70 



1. Sex-inversion in Teleosts 70 



2. Sex-inversion in other Vertebrates 71 



3. Origin of Definitive Germ Cells 7 2 



XI. Summary and Conclusions 73 







I. INTRODUCTION. 



In the fall of 1920, Dr. A. W. Bellamy, who is carrying on 

 breeding experiments with several viviparous teleosts in this labo- 

 ratory, obtained two Xiphophorus hcllcri from F. E. Boehn, a 

 member of the Chicago Aquarium Club, who claimed that these 

 fish had been subject to sex-inversion. According to this report 

 he had three unusually large specimens two females and one 

 male which he kept in a breeding tank and from which he raised 

 more than five hundred young. When the females were about 



