SEX-RATIOS AND SPERMATOGENESIS IN THE 

 TOP-MINNOW, GAMBUSIA HOLBROOKI GRD. 



S. W. GEISER, 

 WASHINGTON' UNIVERSITY, SAINT Louis. 



(From the Zoological Laboratory of the Johns Hopkins University.) 



CONTENTS. 



AGE 



I. Introduction 175 



Sex-ratios Reported in Gambitsia and Other Poeciliids 1/5 



Sex-ratios in Fishes 179 



II. Spermatogenesis in Ganibusia, with Special Reference to its Bearing 



on the Explanation of the Sex-ratio l8l 



Materials 182 



Methods 182 



The General Anatomy of the Testis 182 



Seasonal Variation in the Adult Testis 185 



The Spermatozeugmata 185 



Observations on Spermatogenesis 185 



(a) Primary and Secondary Spermatogonia 185 



(&) Later Stages in Maturation: Leptotene, Synapsis, Pachytene. . . 1 88 



(c~) Tetrads and Diakinesis 188 



(a") Spermatozoa 189 



(e) Dimorphism of the Spermatozoa 189 



(/) Summary on Spermatogenesis 190 



III. Observations on Sex in Ganibusia Raised in Aquaria, with Special 



Reference to its Bearing on the Problem of Sex-ratios 191 



Observations on the Sex-ratios of the Aquarium Populations 191 



Criteria of Sex in Young and Adult Gambitsia 192 



The Gonopod and its Development 192 



The Position of the Anal Fin as a Criterion of Sex 194 



Methods and Results ig6 



The Chromosomal Constitution of the Poeciliidae, and its Bearing on 



the Sex-ratio 200 



A Differential Death-rate in Ganibusia 2OI 



IV. Conclusions 203 



I. INTRODUCTION. 



Sc.v-Ratios Reported in Gambitsia and other Poeciliids. Many 

 observations have been recorded of the sex-ratios in field col- 

 lections of Ganibusia and other viviparous members of the family 



