144 SENESCENCE AND REJUVENESCENCE 



for the answer to this inquiry. The meganucleus of the infusoria 

 is apparently a speciaHzed vegetative organ of the cell not found in 

 the same form in other cells, although Goldschmidt ('05) has 

 attempted to show that all animal cells are physiologically if not 

 morphologically binucleate and that a distinction between vegeta- 

 tive or somatic and reproductive nuclear substance must be made. 

 Whether or not we accept this view, the meganucleus is evidently 

 a specialized organ, and all the facts indicate that it plays an impor- 

 tant role in the metabolic activity of the cell. In the process of 

 division it apparently undergoes no great degree of reorganization, 

 but is merely separated into two parts and continues to grow. If 

 the successive divisions of the meganucleus do not balance the 

 progressive changes between divisions, it will necessarily undergo 

 progressive senescence, and if no other method of rejuvenescence 

 occurs, death from old age must finally result. 



This, I believe, is what actually occurs. The period from the 

 low point of one rhythm to the low point of the next represents the 

 length of life of the meganucleus under the existing conditions. 

 As the meganucleus undergoes senescence after its differentiation 

 as a meganucleus, the rate of growth and division decreases, sooner 

 or later the meganucleus begins to degenerate, and a physiological 

 relation of some sort undoubtedly exists between these changes 

 and the micronuclear divisions which occur. In other words, the 

 process of endomixis is apparently the periodic replacement of a 

 part which has grown old by a new, young part and is therefore 

 analogous in certain respects to the replacement of differentiated 

 old cells by young in the multicellular organism. Like such cells, 

 the meganucleus apparently does not undergo rejuvenescence but 

 dies of old age and is replaced by a new one. 



Further investigation will probably show that the length of 

 time between two successive endomixes may, like many other 

 senescence periods, be altered and controlled experimentally to a 

 greater or less extent. It is in fact possible that under certain con- 

 ditions the degree of rejuvenescence occurring in the ordinary 

 divisions may be sufficient to maintain the race without progressive 

 senescence of the meganucleus and so without endomixis, although 

 it may be that the rejuvenescence in division is rather cyto- 



