Detcriniiiation of Sex in Hydatina serita 15 



Twelve other female-laying females, each of which laid 35 to 39 

 (average 36+) eggs, produced all their male-laying daughter- 

 females among the first twenty-four eggs laid. This is a clearer 

 case than Diagram i, because there are no scattering male-laying 

 females among the later produced eggs. Of the daughter-females 

 16 + per cent were male-laying females. 



In these two diagrams the mother-individuals were not specially 

 selected but the record of all mothers, in Table I, producing 40 

 to 44 daughter-females, is shown in one diagram and the record of 

 all mothers, in Table I, producing 35 to 39 daughter-females is 

 shown in the other diagram. The numbers 35 to 39 and 40 to 44 

 were chosen because they seemed more likely to be the normal 

 than a higher one. 



These results, together with those obtained at different tem- 

 peratures throw a great deal of light upon Maupas' results. In 

 his experiments the highest percentages of males was always 

 obtained from mothers which developed from early laid eggs. 



In Table I it is seen that an individual mother may produce 

 o to 40 + per cent of daughter male-laying females. This fact 

 must also be taken into account when explaining Maupas' few 

 experiments. 



VI INFLUENCE OF FOOD 



Nussbaum supported Maupas' conclusions that external factors 

 can change the sex ratio in Hydatina but explains this change as 

 being due to poor nutrition of the females and not due directly 

 to the influence of temperature. At the higher temperature the 

 processes of metabolism are taking place so rapidly that the ani- 

 mals cannot eat food and assimilate it fast enough to prevent their 

 tissues from being in a semi-starved condition. Nussbaum's 

 experiments seemed to show evidence that young females which 

 are starved for several hours as soon as they leave the egg produce 

 a higher percentage of males than those that are fed from the 

 moment they hatch. 



In many of his experiments he kept many individual females 

 together and did not follow the history of each individual sepa- 

 rately. 



