Life Cycle of Hydatnia Senta 313 



eggs may also appear provided the }oung females copulate within 

 a few hours after hatching. The winter eggs are recognizable 

 by their large size, and thick shell which is covered with many 

 processes like the nap or pile of a fabric. The parthenogenetic 

 eggs have thin shells; those yielding females are usually larger 

 than those hatching into males, but eggs of intermediate size may 

 be of either sex. 



Most of the above facts were reported by Maupas ('90a). 

 In later investigations, the same author (Maupas ('90b) found 

 that when young females were given every chance to copulate, 

 no males were produced and about the same percentage of females 

 produced winter eggs as produced males when copulation was pre- 

 vented. From this he concluded that the winter or resting eggs 

 are fertilized male eggs, and that young females destined to pro- 

 duce females can not be fertilized. Maupas ('91) also performed 

 experiment from wh ch he concluded that the proportion ot male- 

 producers depends on temperature. The offspring of five fe- 

 males kept at 26° to 28° C. included 97 per cent of male-producers, 

 while five sister females at 14° to 15° C. yielded only 5 percent of 

 male-producers. Five other females which were kept in the cold 

 while they laid the first half of their output of eggs, and at 26° 

 to 28° C. while laying the last half, yielded 24 per cent of male- 

 producers in the former lot, and 81 per cent in the latter. Six 

 other females were alternated between high and low tempera- 

 ture, and the highest percentage of male-producers came from 

 eggs laid at the high temperature. 



The possible influence of temperature was afterwards examined 

 by Nussbaum ('97), who got only negative results with tem- 

 perature differences. Nussbaum's experiments showed, he thought, 

 that starvation increased the proportion of male-producers. He 

 tried to reconcile Maupas's findings with his own a"s follows: Mau- 

 pas probably did not isolate the young rotifers as they hatched, 

 so that his aquaria soon came to contain many individuals. At 

 the h'gher temperature, the animals multiply so much more rapid- 

 ly and each one eats so much more, that the quantity of food put 

 into the dishes at the outset soon became exhausted. The en- 

 suing starvation, Nussbaum supposed, effected the increase in 



