Determination of Sex in Hydatina senta 13 



females produce fewer and fewer eggs while the decrease in the 

 number of eggs produced by male-laying females is not as great. 

 Table IX shows a rough approximation of the ratio in which the 

 male and female eggs are produced at these different tempera- 

 tures. 



TABLE IX 



The approximate ratios in which the males and females are produced at different temperatures, 

 as seen in Tables V-VIII. 



From the foregoing experiments and tables it is evident that 

 temperature has nothing to do directly with determining sex in 

 Hydatina senta but indirectly it determines the number of each 

 sex produced by regulating the number of eggs that each kind of 

 female lays. At a temperature of 20° to 22° C. the male-laying 

 and female-laying females lay about the same number of eggs 

 each, but at a higher temperature of 26° to 29° C. the male-laying 

 females lay about four times as many as the female-laying females. 



V EARLY PRODUCTION OF MALE-LAYING FEMALES IN A FAMILY OF 



DAUGHTER-FEMALES 



None of the previous workers with Hydatina senta have iso- 

 lated the eggs of a female-laying female in the order in which they 

 were produced to determine whether there is any tendency for 

 the earlier laid eggs to produce more male-laying females than the 

 later laid eggs. 



In my experiments in which all the daughter-females of each 

 individual mother were carefully isolated and the sex character of 

 their immediate offspring was recorded it is clearly shown that 

 the male-laying daughter females appear among the earlier ones 

 in the family rather than among the later ones. 



In Diagram i the plotted line indicates the production of male- 

 laying females among the 472 daughter-females of eleven mother- 



