258 ANIMAL BIOLOGY 



Suggestive as is this comparison and contrast — and it is not 

 without some justification — the cardinal fact remains that recent 

 work has demonstrated that Paramecium and some closely related 

 forms, when bred under favorable environmental conditions, can 

 continue reproduction indefinitely, at least in one case for more 

 than thirty years and some twenty thousand generations, without 

 fertilization and without any signs of degeneration. Moreover in 

 many unicellular forms fertilization has never been observed and 

 perhaps does not occur in the life history. In other words, fertiliza- 

 tion is not a necessary antidote for inherent senescence, and this, 

 taken in connection with other data which point in the same direc- 

 tion, such as the unlimited reproduction of many plants by asexual 

 processes, and the recent discovery that certain tissue cells re- 

 moved from the Vertebrate body will live, grow, and divide appar- 

 ently indefinitely if given favorable conditions, renders it fairly 

 safe to make the general statement that senescence is not inherent 

 in protoplasm - - the need of fertilization is not a primary attribute 

 of living matter. Reproduction and fertilization are intrinsically 

 separate processes which, however, have become closely associated, 

 especially in higher forms. 



So far our conclusion is entirely negative — fertilization is not 

 reproduction and is not intrinsically necessary for reproduction. 

 What then is its significance? Though fertilization may not be 

 necessary in the life of simple organisms under favorable condi- 

 tions, this does not indicate that it may not be a stimulus to proto- 

 plasmic activity when it does occur — perhaps a very important 

 factor under special environmental conditions. Indeed it appears 

 certain that conjugation in many cases directly results in stimulat- 

 ing the vital processes of the cell, including reproduction. But it 

 would seem that the essential factor in this stimulation is not 

 the essence of fertilization, which is synkaryon formation. In 

 Paramecium, for example, an internal nuclear reorganization proc- 

 ess known as endomixis occurs periodically, which is carried on 

 by each individual, without a nuclear contribution from another. 

 Nevertheless it frequently effects a physiological stimulation similar 

 to that which follows synkaryon formation during fertilization. 

 Accordingly the factor common to both fertilization and endomixis, 

 that is general nuclear reorganization, apparently is responsible 

 for the 'dynamic' effects. (Fig. 171.) 



Metazoa. Turning from Paramecium and its allies, we may 



