200 INDIVIDUALITY IN ORGANISMS 



plants, however, and doubtless in some animals, the 

 symmetry gradients are determined in later stages. 



From this point of view the chief difference between 

 agamic and gametic reproduction is that in the latter 

 the mere isolation of the reproductive body from the 

 parent individual is not sufficient to start the process of 

 dedifferentiation and new development. The gametes 

 do not react except under special conditions, because 

 they have become so highly specialized and differentiated 

 as parts of the parent individual that they are incapable 

 of such reaction. But when the special conditions are 

 present, dedifferentiation begins and development pro- 

 ceeds. Certain eggs develop parthenogenically, and 

 these in many cases are very evidently less highly differ- 

 entiated than eggs which require fertilization. It is 

 probable that they or some of them represent a stage 

 in gametic development in which the egg is still capable 

 of reacting to isolation like the physiologically or physi- 

 cally isolated part of the body of Tuhidaria or Planaria 

 by undergoing dedifferentiation and a new course of 

 development. If this conclusion is correct, these par- 

 thenogenic eggs represent a condition intermediate be- 

 tween the parts of the body of lower forms which 

 undergo agamic reproduction when isolated and the 

 more highly specialized gametes for which fertiliza- 

 tion is a necessary condition of further activity. At 

 least many of the eggs in which development can be 

 initiated experimentally by other means than fertili- 

 zation are apparently almost capable of natural par- 

 thenogenesis, and so are probably less highly specialized 

 than eggs which are not susceptible to experimental 

 treatment. 



