296 SENESCENCE AND REJUVENESCENCE 



at least under the usual conditions, regression does not bring the 

 cell back to a completely undifferentiated stage. 



It is of course true that in some tissues, such as the skin, the 

 more highly differentiated cells show no capacity for dedifferentia- 

 tion, but die and are replaced by division and growth of cells which 

 remain throughout life in a more or less embryonic condition. In 

 such cases there is no evidence of regression and dedifferentiation, 

 but its absence in the one tissue does not justify the conclusion that 

 it is absent in another. DedilTerentiation and regression in tissue 

 cells are undoubtedly associated with rejuvenescence in the higher 

 as well as in the lower organisms, and tissue regeneration, whether 

 a feature of normal life or the result of injury, must bring about 

 some degree of rejuvenescence in the parts concerned. 



After a period of hibernation, tissue regeneration is often very 

 extensive (Monti, '05) and may involve tissues which usually show 

 but little regeneration. In such cases the large proportion of young 

 cells in the body must render the animal as a whole, though not 

 necessarily all parts of it, appreciably younger than before hiberna- 

 tion. In fact, the periodic cycle of activity and hibernation in 

 various forms is in many respects similar to an age cycle. It is 

 probable that the rejuvenescence begins during the hibernation 

 period when the animal is living upon its own substance, like the 

 starving planarian, and that this change leads sooner or later to 

 renewed division and growth of cells. At the same time, other 

 cells doubtless die and are later replaced by the younger cells. 



Other periodic changes, particularly in the glandular tissues, 

 show the essential characteristics of an age cycle. In the pancreas 

 cell, for example (see pp. 189-191), the loading of the cell is both 

 morphologically and physiologically similar to senescence, and the 

 discharge to rejuvenescence. In such cases the changes occur in 

 individual cells without cell reproduction. 



The cells of the nervous system in man and many animals are 

 believed to persist throughout life, and to possess no appreciable 

 capacity for regression and dedifferentiation beyond their abihty 

 to regenerate the nerve fibers which arise from them. Doubtless 

 this belief is correct, so far as visible structural changes or measure- 

 able metabolic changes are concerned; but is there not reason to 



