160 A. FRANKLIN SHULL 



ation of the sexual individuals with certain broods may have 

 been brought about. 



This conclusion of Weismann's was not openly challenged, so 

 far as I can find, until Issak6witsch ('05) reported that he was 

 able to increase the number of sexual individuals by starvation, 

 and, indirectly, by low temperature. He went so far as to claim 

 that a cycle, in Weismann's sense of the word, does not exist 

 among daphnians. He recognized, however, that the longer a pure 

 line was prevented from producing sexual forms, the greater was its 

 tendency to do so, which maybe a slight concession to Weismann's 

 view. The conclusions of Issak6witsch were rejected by Keilhack 

 ('06) and Strohl ('08) on insufficient grounds, it seems to me, while 

 later workers have confirmed the conclusion that external agents 

 are effective in determining the life cycle of certain daphnids. 

 Langhans ('09) finds that the excretions of the daphnids them- 

 selves constitute such an agent; and Woltereck points out that 

 different species and different local races behave differently in 

 this regard, some species and pure lines responding readily to 

 external conditions, others responding but slightly or having their 

 cyle determined almost solely by internal factors. In some species, 

 he finds, it is plain that both sets of agents are effective. McClen- 

 don ('10) likewise states that starvation, temperature differences, 

 and excretions modify the life cycle, but he was unable wholly to 

 exclude sexual forms in some cases. Papanicolau ('10) states that 

 both internal and external conditions affect the life cycle, but not 

 at all times in the cycle. In the two genera with which he worked, 

 he recognized three periods in each cycle which differed in the sus- 

 ceptibility of the animals to external conditions. There is a purelj^ 

 parthenogenetic period, he says, comprising the first generation, 

 counting from the resting egg, and the first broods of some later 

 generations. The eggs laid in this period are, in the genus Moina, 

 violet in color, and the females that hatch from them can not be 

 made, by any external conditions,to show a sexual tendency. 

 Another period, Papanicolau states, comprises the late broods of 

 late generations, in which the females, which in Moina hatch from 

 blue eggs, have a sexual tendency, and no external conditions can 

 make them parthenogenetic. Between these is a period which 



