392 SENESCENCE AND REJUVENESCENCE 



generations, which corresponds to the aging and the .attainment of 

 sexual maturity in other forms. The parthenogenic female- 

 producing egg is apparently characteristic of the young animal 

 and the earlier generations in a cycle, the parthenogenic male- 

 producing egg and the zygogenic female-producing egg of a more 

 advanced age in the individual and in the generations. Richard 

 Hertwig ('i 2) in discussing these facts says : " It is therefore possible 

 to speak in a double sense of an aging of the daphnids and of a 

 change in the constitution of the eggs determined by it." 



Woltereck has found that an individual may pass through 

 more than one of these reproductive cycles. Even after producing 

 winter eggs, females may again pass through a labile period in 

 which the character of the eggs can be influenced by external con- 

 ditions and still later attain a condition in which the eggs are 

 parthenogenic and female-producing in spite of external conditions. 

 In other words, they become physiologically young again. But it 

 has been shown in earlier chapters that such rejuvenescence occurs 

 in many forms. In the case of the daphnids it does not proceed 

 as far as in many of the lower animals, for these may lose their 

 sexual organs entirely and return to reproduction by budding or 

 fission, while in the daphnids we find only a return from the produc- 

 tion of zygogenic to the production of parthenogenic eggs. 



As regards the rotifers, in certain species of which partheno- 

 genesis and bisexuality exist, the various investigators 1 still differ 

 widely in their opinions as to the determining factors. The effect- 

 ive factor in determining parthenogenesis and bisexuality is accord- 

 ing to Maupas temperature, and according to Nussbaum nutrition, 

 while Punnett finds that neither of these external factors is con- 

 cerned, but that the character of the eggs is hereditarily determined. 

 Whitney regards the age of the family, that is, the position in the 

 cycle of generations, as the important factor, although he admits 

 the influence of external conditions. And, finally, Shull has demon- 

 strated the influence ef external factors in the environmental 

 medium, apparently of chemical nature, but believes that internal 

 factors are also involved. With such differences of opinion it 



1 Maupas, '91; M. Nussbaum, '97; Punnett, '06; Shull, '10, 'na, 'nb, '12; 

 Whitney, '07, '120, '126. 



