Page:Popular Science Monthly Volume 64.djvu/107

Rh 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 egg and the deposition of her first egg, determines 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 temperature 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. Nussbaum 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 summer 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 characteristic 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 condition 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 parthenogenetic 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 Phyloxera vitifolii, which is parasitic on the