THE DETERMINATION OF SEX. 103 



produce males. A third kind of female produces the winter eggs, 

 which are fertilized by the males and give rise to females. In this 

 rotifer the sex of the egg is determined while the egg is still in the 

 ovary, and Nussbaum has made the important discovery that the 

 amount of nourishment taken by a young female, between the time of 

 her emergence from the Qgg and the deposition of her first egg, deter- 

 mines which kind of eggs she will subsequently produce. If she has 

 been well nourished in this interval she produces eggs that become 

 females, but if poorly nourished she produces male eggs. After the 

 eggs have been once formed no subsequent change of food or of tem- 

 perature can alter the kind of eggs that are produced. It has not been 

 determined why some females produce parthenogenetic eggs and other 

 females winter eggs that are to be fertilized. Nussbaimi thinks that 

 the effect of an early union with a male, combined with insufficient 

 nourishment during the first hours of free life, determine that winter 

 eggs are to be produced. 



Amongst crustaceans and insects there are several instances known 

 in which the sex of the individual appears to be connected with certain 

 kinds of eggs. The water fleas, or daphnids, produce during the sum- 

 mer small parthenogenetic eggs with a thin shell which develop into 

 parthenogenetic females,* but under certain conditions males and 

 females appear. The females produce large winter eggs which are 

 fertilized and produce in the following year only female daphnids which 

 start the parthenogenetic summer broods. The sex of the winter eggs 

 is probably determined in the ovary, since the eggs show their charac- 

 teristic structure before they are set free. The appearance of the male 

 and female generation is supposed to be connected with the change in 

 temperature, or more probably with a change in the amount of food. 

 Under these conditions, as has just been said, eggs that produce males 

 and females are formed. Here it would appear that an external con- 

 dition determines the appearance of the male and of a different kind 

 of female. 



Similar facts are known for the aphids, or plant-lice. If conditions 

 are favorable, i. e., if they are kept warm and have an abundance of 

 succulent food, they continue indefinitely producing wingless parthen- 

 ogenetic females. But if the food becomes scarce or dry, then winged 

 males and females arise from the parthenogenetic eggs; these unite, 

 and the fertilized winter eggs are laid. From these eggs the wingless 

 parthenognetic females arise in the following spring. 



The life history of Pliyloxera vitifolii, which is parasitic on the 



* Lenssen claims that the parthenogenetic female eggs do not give off a 

 polar body, and that the male eggs give off only a single polar body. Whether 

 this difference may have any relation to the sex of the individual will be dis- 

 cussed later. 



