CONDITIONS OF GAMETE FORMATION 



391 



Woltereck's extensive investigations, together with the evidence 

 from the work of others, seem to show very clearly that, while 

 differences exist in different races and species, nevertheless a cyclical 

 change affecting the character of the eggs produced does occur in 

 these animals independently of external factors, although it may 

 be modified by temperature, nutrition, and chemical constitution 

 of the medium in which the animals live. Woltereck divides the 

 cycle into three periods, the first including the early generations of 

 females following the winter eggs. These females are predomi- 

 nantly parthenogenic and female-producing, at least until late in 

 life, and external factors have no effect on the character of the eggs. 

 After this follows a second period in which external conditions 

 determine to a very large extent whether parthenogenic eggs pro- 

 ducing females, or parthenogenic eggs producing males and zygo- 

 genic eggs are produced, and finally a third period occurs in which 

 parthenogenic eggs producing males and female-producing zygogenic 

 eggs appear independently of external conditions. 



Von Scharfenberg and Papanicolau found that a change in egg 

 character occurred, not only in the course of successive generations, 

 but also in the course of single generations, i.e., the eggs produced 

 early in the life of a female are more Hkely to develop partheno- 

 genically into females and those produced later in Hfe into males 

 or to be zygogenic winter eggs. In the earlier generations of a 

 cycle the male-producing and zygogenic eggs appear later in the 

 life of the individuals, in later generations earlier. Moreover, 

 the same three periods appear more or less clearly in the repro- 

 ductive cycle of the single females as in the cycle of generations. 



This reproductive cycle appearing both in single individuals 

 and in successive generations is in certain respects analogous to the 

 cycle of agamic and gametic reproduction in many of the lower 

 animals. In the early stages of the cycle the daphnids, although 

 producing what we call eggs, are really reproducing agamically, 

 since the eggs develop parthenogenically, but in later generations, 

 as well as later in the Hfe of the individual, they become sexually 

 mature, and males and females appear and the eggs require ferti- 

 lization. There seems to be a progressive change in physiological 

 condition in these animals, both individually and in successive 



