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 suppHed 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, i.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 40 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 

 tow^ard showing that external conditions regulate the hfe-cycle 

 of the daphnias. 



Issakowitsch kept a species of Daphnidae, 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. 



