Life-cycle of the Rotifer ; Hydatina Senta 349 



cold and laid there 34 eggs, of which 12 per cent only became 

 individuals that laid male eggs and '^'^ per cent individuals that 

 laid female eggs. The same parents were then transferred to 

 a warmer temperature and laid 95 eggs that were male pro- 

 ducers and 5 eggs that were female producers. They were changed 

 back and forth from warmth to cold, and each time a correspond- 

 ing change occurred in the individual that came from the eggs. 

 During the last four days of their hfe they produced 50 eggs, 

 of which 17 per cent were male-producing and Z-^ per cent 

 female-producing. 



Nussbaum has tried to show that the sex of the offspring is de- 

 termined by the environment and not by influences on the egg. 

 If the young parthenogenetic female after she has emerged is 

 well fed, she produces large eggs that develop, without fertiliza- 

 tion, into female individuals. If on the other hand an individual 

 is at first poorly fed, she produces small eggs, and these develop 

 without fertihzation into males. Before and after this period 

 the sex of the egg is not determined by the food supply, i.e. after 

 the first egg has been laid it is no longer possible to change the 

 sex of the other eggs. 



Nussbaum thinks that Maupas was mistaken in regard to the 

 influence of temperature, because he used too many individuals, 

 whose numbers in proportion to the food supply determined 

 the conditions of nutrition that exist. Maupas may also 

 have judged the sex of the offspring from the size of the egg. 

 This is not always possible, according to Nussbaum ; for while 

 it is true that the biggest eggs become females, and the smallest 

 males, there are many of each kind whose sizes overlap. 



Temperature favors the appearance of the males in so far as 

 each animal hves at a higher rate at a higher temperature and 

 needs more food, also at the higher temperature the output of eggs 

 is faster, hence the food conditions poorer. At a lowxr tempera- 

 ture the output is slower and food conditions are relatively better; 

 but since the kind of ^gg produced is determined before the first 

 egg is laid, it is not obvious that this criticism is apropos. 



Plenty of food during the development of the ovary leads to 



