CONDITIONS OF GAMETE FORMATION 393 



seems at least probable that internal physiological conditions, 

 which are not yet clearly recognized, are the real determining 

 factors, and that the various external factors merely modify their 

 action. As Woltereck ('n) has pointed out, there is every reason 

 to believe that the relation between parthenogenesis and bisexu- 

 ality is essentially the same in the rotifers as in the daphnids. 



Parthenogenesis and bisexuality are also found among the 

 plant lice and various other related forms among the hemiptera. 

 In these cases, as in the daphnids, bisexuality appears later in the 

 cycle than parthenogenesis, but concerning the influence of external 

 conditions in modifying the usual course of events, our knowledge 

 is fragmentary. Low temperature or lack of food may apparently 

 at times induce bisexuality, as in the daphnids. All that we know 

 suggests that in this, as in other cases, bisexuality is a feature 

 of more advanced age than parthenogenesis, and that the aging 

 may be accelerated or retarded, perhaps reversed, by external 

 conditions. 



The parthenogenic egg is apparently a less highly specialized, 

 physiologically younger cell than the egg requiring fertilization. 

 Morphologically it is less highly differentiated, at least in many 

 cases, than the zygogenic egg (see pp. 342-46), and when isolated 

 from the parent body it is capable of developing at once without 

 fertilization (cf. pp. 406-8). If such eggs are produced by animals 

 in the earlier stages of their adult life history or by the earlier 

 generation of a cycle, we are forced to the conclusion that the germ 

 cells undergo differentiation and aging like the rest of the body. 

 In short, the egg produced by the older organism is itself older 

 and more highly specialized than that produced by a younger. 



The physiological character of the action of external conditions 

 in modifying the eggs can at present only be surmised. Woltereck 

 suggests that the differences in the eggs are due to differences in the 

 intensity of assimilation in the ovary, high intensity determining 

 parthenogenic female-producing eggs and low intensity bisexual 

 eggs. A decrease in intensity of assimilation is, however, a char- 

 acteristic feature of senescence and may result from internal as 

 well as from external conditions. Apparently the external factors, 

 whatever the exact mechanism of their action, either accelerate, 



