342 Experimental Zoology 
egg. It is interesting to note that if the winter egg is not fer- 
tilized it does not develop parthenogenetically, although it is 
better supplied with food than the parthenogenetic egg. Thus 
while there is a sharp distinction between the two kinds of eggs, 
there is no such distinction between the females that produce 
them; for the same female may first produce one sort and then 
the other. If external conditions determine which kind of re- 
production is to take place, they do so by affecting the kind of egg 
rather than the kind of individual; but it is interesting, even here, 
to notice that the conditions that call forth the males are those 
that call forth the production of the winter eggs in the females. 
The winter egg is also a resting egg, 7.e. although after fer- 
tilization it passes through the cleavage stages its further de- 
velopment is there arrested, even if placed in water at a favorable 
temperature. In some species it has been found that the rest- 
ing eggs will not develop further unless they are first dried *— 
an excellent adaptation for preserving the race, for the eggs re- 
main undeveloped until the pool becomes dry, and later may 
stock the pool again with a new brood. In other species, in 
Moina and in Daphnia, according to Weismann, the resting eggs 
will develop after a time if kept continuously in water. The 
resting eggs of Moina paradoxa took from 30 to 4o days to 
develop in June, but in September only 10 days. For Daphnia 
pulex, the egg took from 18 to 31 days in the spring, and from 
47 to 60, or even to 80, days in September. 
A series of experiments have been recently carried out by 
Issakowitsch, under the direction of R. Hertwig, that go far 
toward showing that external conditions regulate the life-cycle 
of the daphnias. 
Issakowitsch kept a species of Daphnide, Simocephalus 
vetulus, at different temperatures and obtained different kinds 
of individuals according to the temperature employed. For 
instance: a parthenogenetic female was isolated and kept at 
24°C. Between February 6 and April 5, 6 generations appeared 
? Brauer found that the eggs of Apus will not develop unless first dried, and 
Spangenberg found that this is also true for Branchipus. 
