SEX-RATIOS IN GAMBUSIA HOLBROOKI. 



this is owing to peculiarities in the distribution of possible sex- 

 determining- chromosomes, or to a differential viability in the 

 gametes, or the zygotes, both in embryonic and juvenile stages. 



II. SPERMATOGENESIS IN Gambusia, WITH SPECIAL 



REFERENCE TO ITS BEARING ON THE 



EXPLANATION OF THE SEX-RATIO. 



The illuminating genetic studies of Schmidt ('20) have demon- 

 strated that Lebistes has an XX, XY constitution. Aida ('21) 

 has also shown the same thing to be true of another Pceciliid fish, 

 Aplocheilus. Since Gambusia and Lebistes are closely related 

 it is reasonable to assume that the chromosomal arrangement in 

 these two forms is similar. If this is true, then we should ex- 

 pect in Gambusia equal numbers of potential males and females 

 at the time of fertilization, and if this is what actually obtains, 

 then the preponderance of females 'observed in adult populations 

 must be due to a differential death-rate operating after fertiliza- 

 tion. The importance of the investigation of spermatogenesis. 

 in this form is consequently obvious. 



But little work has been done on the spermatogenesis of teleost 

 fishes. The only title that the writer has been able to find is 

 that by Turner ('19) on the seasonal cycle of the testis of the 

 perch. 3 Bohm ('91) and Behrens ('93) have investigated the 

 oogenesis of the " Forrelle " and Blanc ('94) has studied the 

 oogenesis of Trutta lacustris. Mrs. Harvey ('20) in her 

 synoptic paper has noted brief observations by other authors 

 on diploid cleavage numbers in a number of other teleost fishes. 

 But except for the paper of Turner above noted no study appears 

 to have been made of the male sex-cells of teleosts. The ex- 

 planation seems to lie in the fact that in this group of fishes the 

 chromosomes are very numerous, small in size, and lack indi- 

 viduality. 4 



3 Since the above was written, Winge ('220} has briefly described the 

 spermatogenesis and oogenesis of Lebistes reticulatns, and Essenberg ('23) 

 has published brief observations on spermatogenesis in Xiphophorus, a poeciliid 

 fish. 



* In Lebistes and Xiphophorus the same difficulty is found. (Cf. Winge. 

 '220, and Essenberg, '23.) 



