SEX-RATIOS IN GAMBUSIA HOLBROOKI. 185 



prophases. Along the median plane there also occurred a band 

 of cysts whose cells were in the same stage of mitotic division 

 as those of the cysts occurring at the periphery of the testis. 

 The line of fusion of the two original testes appears clearly in 

 the vertical band of younger cysts running through the section 

 figured. 5 



Seasonal Variation in the Adult Testis. The testis shows a 

 marked seasonal periodicity in volume, the number of contained 

 spermatozeugmata, and the volume of the longitudinal testicular 

 canal. During the spring and summer, which is the breeding 

 season, the testis increases in size. The increase is, however, 

 not large, as it averages only 60 per cent., while the maximum 

 is only a sixfold volume-increase. This is much less than has 

 been found for other fishes, e.g., the perch. 



During the late autumn and winter and very early spring 

 the testis is filled with spermatozeugmata which greatly distend 

 the longitudinal testicular canal. After the great spring wave 

 of copulation the testes show very few spermatozeugmata, and 

 in June and July the testes are filled with early stages of sper- 

 matogenesis. Smears of the testes taken in August and Sep- 

 tember show only spermatids and spermatozeugmata along 

 with late spermiogenesis stages. 



The Spermatozeugmata. When ripe, the spermatozeugmata 

 are spheroid to ovoid bodies with a diameter in winter testes of 

 210-320 micra. They lack completely the investing membrane 

 characteristic of spermatophores and agree entirely in structure 

 with those, described for Cnesterodon lo-maculatus by Philippi 

 ('08.) It is probable that the error into which Kuntz ('14) has 

 fallen with regard to the structure of the " spermatophore " is 

 due to his having studied and described cysts containing imma- 

 ture spermatozoa. For in mature cysts and spermatozeugmata 

 the spermatozoa lie at the periphery and have a definite arrange- 

 ment. 



Observations ( on Spermatogcnesis. (a). Primary and Sec- 

 ondary Spermatogonia. The definitive sex-cells by repeated divi- 

 sions give rise to the final spermatogonia. It is not known how 



5 Essenberg's " Figure 38 " also shows very well the similar bilaterality of 

 the testis in the related species, Xiphophorus helleri. 



14 



