Factors Determining the Annual Distribution. 861 
gust, and the decline continued steadily through September. 
Similar conditions of temperature to those of 1896 were found 
in 1894. 
There is no fall reproductive season for Diaptomus , but as 
the temperature declines the number of egg-bearing females 
diminishes, and the number of individuals of the species be¬ 
comes steadily smaller. The winter level is reached compara¬ 
tively early, in late October or the very first of November. 
After this level is reached, no increase takes place until after 
May 1st of the following year. The number however, remains 
singularly constant throughout the winter, and the individual 
members are well nourished, containing large quantities of fat 
at all times during the winter. 
Daphnia hyalina has two great periods of reproduction, in tho 
spring and fall. The ovaries begin to develop before the ice 
has disappeared from the lake in late February and in March,, 
when the temperature of the water is 2.5° C., or above. 
A very few of the largest individuals produce eggs at this time,, 
but no considerable number of eggs are found until the temper¬ 
ature of the lake reaches 4-4.5° C., which has been about 
the middle of April. In 1895 the first numerous broods of young 
Daphnias appeared about the middle of May, when the upper 
water of the lake had reached an average temperature of about. 
15° C., and the reproductive period lasted until about the 
middle of June. During this time the number of eggs is con¬ 
siderable, usually as many as five and occasionally nine, or 
even more. These eggs are smaller than those produced in the 
summer, the yolk is peculiar in color, and in general the eggs 
resemble more nearly those of the ephippia than the eggs pro¬ 
duced in midsummer. About three broods are produced during 
the month by the females. Toward the end of this reproduc¬ 
tive period males appear in small numbers. They never exceed 
4 per cent, of the total number of the females, and I have never 
found ephippial females at this season though I have searched 
carefully for them. 
During the first part of June those females die which have 
lived through the winter, and at the same time there seems to be 
a break in the reproductive activity of the species. Whether 
