SEX-RATIOS IX GAMBUSIA HOLBROOKI. 191 



distribution that would explain the occurrence of the atypical 

 sex-ratio observed in the adults. 



III. OBSERVATIONS ON SEX IN Gambusia RAISED IN AQUARIA 



WITH SPECIAL REFERENCE TO ITS BEARING ON THE 



PROBLEM OF SEX-RATIOS. 



Observations on the Sex-Ratios of the Aquarium Populd'- 

 tions. During the years 1919-1920 a series of experiments with 

 gravid Gambusia was conducted with a view to ascertaining 

 whether it were possible in these fish to alter the sex-ratios of the 

 young at birth by the feeding of male gonads to the pregnant 

 females. It was known that the males and females cannot be 

 distinguished by superficial observation, but it was assumed that 

 they could be distinguished by cytological methods. Conse- 

 quently the young of the experimental females were taken at 

 birth, or several days later, fixed in Bouin's fluid, and the entire 

 animal later sectioned and studied. It was found on studying 

 the sections thus obtained that while in some of the individuals 

 unmistakeably definitive oocytes occurred in the usual position 

 just beneath the air-bladder, in others the gonads were still un- 

 differentiated. No internal structures, such as genital ducts or 

 other features gave an indication of the sex. It was conse- 

 quently evident that the sex of Gambusia at birth cannot be as- 

 certained by cytological methods. 7 



The next endeavor was to raise the young in laboratory aquaria 

 until the sexes were distinguishable. At birth they cannot be 

 so distinguished. In adults it can be done by superficial ex- 

 amination of the anal fin, or by dissection. At the age of three 

 weeks after birth it can be done by the cytological method, and 

 at the age of three months, if life-conditions are optimum, by 

 the examination of the anal fin. 



7 In this connection, it is interesting to note that since the above was 

 written, Champy & Gley (Arch. d'Anat. microscopique, T. 19, Fasc. 2, p. 259, 

 1923) have found with the poeciliids Haplochilns, Poecilia, Lebistes, Xipho- 

 phorus, and Platypacilus a very tardy development of the ovary of the young 

 fish. They note that the state of maturity of the gonads is not attained until 

 the animal has acquired its adult size, which is here sharply denned, and 

 that the gland remains in an immature state in animals almost as large as the 

 average-sized sexually mature fish. 



