198 THE POPULAR SCIENCE MONTHLY 



Similar conditions of renewed vigor are shown by many animals after 

 long hibernation. The great breeding activity of many animals, such as 

 frogs, so soon after their winter sleep, may find a physiological explana- 

 tion in this using up of metabolic products during hibernation and the 

 subsequent increase in vitality. 



In similar manner it is known that the new tissue which is formed 

 in regeneration comes from undifferentiated (epithelial or lymphoid) 

 or from dedifferentiated cells (e. g., muscle cells of amphibia, etc.). 

 In the latter case also there is a rejuvenescence, due to the loss of differ- 

 entiation products. In this case dedifferentiation is evidently due, in 

 the first instance, to the injury. It is at least possible that the failure 

 to regenerate lost parts, which many animals show, is due to the inabil- 

 ity of the cells to undergo dedifferentiation and subsequent rejuvenation. 



In conclusion, we find that the life of a cell is dependent upon the 

 continued interaction of nucleus and protoplasm; that as the proto- 

 plasm is transformed into products of differentiation this interaction 

 of nucleus and protoplasm is reduced and constructive metabolism is 

 diminished; that when the quantity of protoplasm present has been 

 reduced beyond a certain point, either by its transformation into meta- 

 plasm, or by other means, constructive processes fail to compensate for 

 destructive ones, and the cell grows old and finally dies. On the other 

 hand, processes which lead to the increase of the general protoplasm in 

 a cell, either by the growth of the protoplasm already present or by the 

 conversion of metaplasm into protoplasm, lead also to the growth of the 

 nucleus, to increased interchange between nucleus and protoplasm, and 

 hence to increased powers of assimilation, cell division and regulation. 

 Anything which decreases the interchange between nucleus and proto- 

 plasm leads to senility; anything which decreases this interchange 

 renews youth. 



