Life-cycle of the Rotifer; Hydatina Senta 347 
in fact, they do not appear at all unless previous union with 
a male has occurred. 
Maupas found that of 796 females isolated from males only 
parthenogenetic eggs were produced (male or female). Of 172 
females to which males had been added at the right time (about 
6 hours after hatching), 84 produced winter eggs and 88 
parthenogenetic eggs (male or female). In another experiment 
822 females were used, and 342 of these were observed to unite 
with males. Of these (342) there were 252 that laid winter eggs, 
the other go produced summer eggs. The ratio of individuals 
laying winter eggs to those laying summer eggs is, therefore, 74 
to 26. The remainder of the 822 females that were not seen to 
pair, 480 in number, gave the following results: 361 produced 
males and 119 produced females. The ratio is 75 to 25. This 
is nearly the same ratio as the ratioof winter eggs to summer 
eggs given above. From this similarity in proportion Mau- 
pas concludes that only male eggs can be fertilized, and these 
become winter eggs. The winter eggs give rise solely to females. 
If this conclusion is established, it follows that fertilization 
changes the sex of the egg, asin the bee. Nussbaum believes the 
similarity in the ratios may be purely accidental, but will not 
deny that the conclusion may be correct. 
Maupas tried to show that temperature is the factor that 
determines the fate of the eggs. If the temperature is lowered, 
females are produced. If the temperature is raised, males are 
produced. He gives the following experiment to establish this 
conclusion. Five immature females were kept at a temperature 
of 26° to 28° C. They laid 104 eggs, of which 97 per cent pro- 
duced individuals that laid male eggs and only 3 per cent 
produced individuals that laid female eggs. 
In the control experiment five immature individuals from the 
same culture were kept at a temperature of 14° to16°C. They 
laid 260 eggs, of which 5 per cent became male and 95 per cent 
became female producers. It will be noticed that in these two 
experiments both the mother and the eggs she produced were 
under the same conditions of temperature, hence the effect 
