S. W. GEISER. 



These experiments endeavoring to raise the young in labora- 

 tory aquaria until the sexes were distinguishable came to naught, 

 for the mortality in indoor aquaria was so great that even with 

 the best of care and feeding the differentiation of the sex-glands 

 did not occur during the maximum period that it was possible 

 to keep any considerable portion of the litters alive. Micro- 

 scopic study of those individuals which had died during the 

 experiments cast no light on the proportions of the sexes of 

 these fish. 



Before proceeding further it may be well to consider the 

 criteria of sex in young and adult Gambusia. 



Criteria of Sex in Young and Adult Gambusia. The vivipa- 

 rous Pceciliidae, such as Gambusia, possess a well-marked sexual 

 dimorphism. The males are much shorter and more slender 

 than the females. The anal fin in the male is placed farther 

 forward than in the female, and is modified into a conspicuous 

 " intromittent " or copulatory organ, the gonopod. This dimor- 

 phism of the sexes is so marked that in one species male and 

 female were originally classified in different genera, and their 

 identity was disclosed only after a lapse of years, when indi- 

 viduals of the assumed two genera were observed to copulate. 

 The sexual dimorphism of the adult constitutes a very service- 

 able criterion of sex, but in juvenile, immature forms it is inade- 

 quate. Studies to ascertain the limits of variation of sex-char- 

 acteristics, such as position of the anal fin in male and female, 

 were therefore made by the writer in order to gain data which 

 would permit one to ascertain readily, without dissection, the 

 sex of an individual suspected of being a " sterile male." A 

 study was also made of the development of the anal fin in the 

 male from the indifferent stage to that of a fully-developed 

 gonopod, for data concerning the time of first appearance of 

 the gonopod, the most striking characteristic of the male, and its 

 degree of development and differentiation in early stages were 

 considered of the greatest interest and importance. A reliable 

 criterion of sex in the early period of life of the fish was much 

 desired. 



The Gonopod and its Development. The modified anal fin 

 in the various groups of Pceciliids becomes a gonopod in accord- 



