SEX-RATIOS IN GAMBUSIA HOLBROOKI. 197 



spectively. Female a had a litter of 37 young; b, of 19; c, of 22; 

 and d, of 24. The birth of all these litters except that of a 

 was observed, and it is known with certainty that none of the 

 young were eaten by the mothers. It is most probable in the 

 case of Female a that none were eaten, for in the writer's stocks, 

 37 is a very good litter. As soon as parturition was completed 

 the mothers were removed. The young were fed on Daphnia 

 for a few days in the small aquaria where they were born ; after 

 which time they were removed and put into concrete aquaria 

 90X160X60 cm. These pools had been thoroughly cleaned out 

 in the spring and stocked with Elodea and Spirogyra. During 

 the summer the Elodea grew until it formed a thick forest of 

 vegetation in the aquaria, offering excellent protection to the 

 young Gambusia. Litter a was put into one of these concrete 

 aquaria, while Litters b, c, and d were combined and put into 

 another. They were left in these aquaria without food, except 

 such as came to them in the form of insects, etc., until the first 

 group was 26 days old, and the second group three and a half 

 months old. They were then removed, killed, and fixed in modi- 

 fied Bouin's fluid. All of the 37 young in Litter a, and 58 of 

 the 65 of the other litters, were recovered. In the aquarium 

 containing the latter there were found in addition to the 58 in- 

 dividuals, 12 which were relatively very small. These twelve 

 were evidently offspring of some of the 65 individuals which 

 had been put into the aquarium in May. 



The proportions of the sexes in Litter a were learned by a 

 histological study of the gonads of all the individuals. The 

 young were taken after fixing was completed, the viscera dis- 

 sected out in a mass, and these sectioned and studied. The 

 gonads in all of these were differentiated to such an extent that 

 the sexes could readily be ascertained by histological examination. 



The sex of the young in the litters which were three and a 

 half months old when killed could be readily be learned with 

 certainty by examining the gonads under a binocular microscope. 

 This was done in all of the specimens except those which had 

 well-developed gonopods and were consequently unquestionably 

 males. It was found that all the males in this lot had developed 

 a gonopod, so that the presence or absence of the gonopod 



