The Biology of Senescence 



which give rise periodically to mictic generations. The literature 

 on sex-determination in rotifers is reviewed by Hyman (1951). 

 The longevity of the two types of female differs considerably; 

 'somewhere in the ontogeny of the females, it must be deter- 

 mined which kind of egg they are destined to lay. The deter- 

 mination occurs during the maturation of the egg from which 



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4 DAY ORTHOCLONE 



II ■ 



17 " 



P Fl 2 3 4 5 



ORTHOCLONE GENERATION 

 Fig. 24. — Life-span of successive generations of Philodina reared in each 

 generation from the eggs of 4- 11- and 17- day-old mothers (Lansing, 1948). 



the female comes, that is, during the last few hours before the 

 egg is laid' (Hyman, 1951). Both internal and environmental 

 factors of great complexity appear to operate in different forms. 

 'The conclusion from numerous researches seems to be that, in 

 addition to an inherent rhythm as regards male production, 

 monotony of conditions suppresses mictic females, whereas any 



90 



