190 Reproduction in Simocephalus vetulus 



Amongst other upholders of the view of an inherent cyclical change 

 which can be influenced to a greater or less extent by environment are 

 Issakowitsch, Papanicolau, Woltereck. The kind of cycle maintained by 

 Woltereck is however quite different from that supposed by Hertwig. The 

 last-named worker correlates it with his nucleo-cytoplasm relation theory 

 and considers that the lapse of parthenogenetic generations and of time 

 leads to a state of depression in the germ cells that eventually under 

 normal conditions ends in sexual conjugation or death. Woltereck justly 

 points out that the theory would have to be strained to breaking point 

 to cover his own experiments with Hyalodaphnia, etc. For instance, 

 Woltereck bred a line of H. cmullata parthenogenetically for four years, 

 during which time it fluctuated between pure parthenogenesis and pro- 

 nounced sexuality. Hence we should have to suppose that the line 

 recovered from pronounced sexual depression periods without sexual 

 conjugation. 



Woltereck's own view is that there are two antagonistic substances 

 in the egg, the predominance of one resulting in parthenogenetic 

 individuals, and of the other in sexual forms. These substances are 

 supposed to wax and wane alternately and autonomously. At such 

 times (labile periods) as they are nearly equally balanced, external 

 conditions are able to turn the scale one way or the other, and thus at 

 times environment influences sex. "Mann kann deshalb die innere 

 zyhlische Periodizitdt der Valenz (of these two substances) mit Recht 

 als das Kernproblem der Cladocerenfortpflanzung bezeichnen" (p. 123). 



Although Woltereck's cycle is far more compatible with the observed 

 facts than is the theory of a depression caused by continued partheno- 

 genesis as such, nevertheless there does not seem to be sufficient evidence 

 for the existence of any sort of cycle at all, while there is strong evidence 

 against it. 



The evidence for the existence of such cycles consists mainly of: 



1. Experiments where the conditions were supposed to be constant, 

 such as (a) Papanicolau's work with Simocephalus and Moina, where an 

 increasing sexuality and degeneration are observed, and (6) Woltereck's 

 with Hyalodaphnia, where the line fluctuated between pure partheno- 

 genesis and a high degree of sexuality. 



2. Experiments where abnormal conditions of various kinds are 

 found to have a determining influence in some cases and not in others. 

 When such conditions are effective it is supposed that it is because the 

 cycle has reached the point where the tendencies to parthenogenesis and 

 sexuality are nearly balanced. When they arc ineffective, it is ascribed 



