CHAPTER XII 



REJUVENESCENCE AND DEATH IN THE HIGHER ANIMtVLS 



AND MAN 



REJUVENESCENCE IN THE LIFE HISTORY 



While much has been written concerning senescence and death 

 in man and the higher animals, but little attention has been paid 

 to the question of the occurrence of rejuvenescence, and many 

 authorities still maintain that Kfe is always a progressive process 

 and that rejuvenescence does not occur. It is of course true that 

 in the higher animals the progressive features of development are 

 predominant and that development ends in death, and many 

 studies of senescence have been based on these forms alone, without 

 consideration or knowledge of the lower organisms. But if we are 

 to reach a general conception of the age cycle in organisms, the wide 

 occurrence and significance of dedifferentiation and rejuvenescence 

 in the lower animals and the plants must at least raise the question 

 whether similar processes do not occur to some extent in higher 

 forms. 



Even in man and the other mammals the different tissues do 

 not undergo senescence ahke. Certain cells, such as the Malpighian 

 layer of the skin, continue to divide and replace the old dying or 

 dead cells of the epidermis, and remain relatively young in appear- 

 ance and behavior throughout the life and even after the death of 

 the individual. In various other tissues such replacement of old 

 differentiated, or dead cells by younger cells occurs more or less 

 extensively in normal life, and tissue regeneration, following injury 

 or loss of tissue cells, occurs to a greater or less extent in all tissues 

 except the nervous system. 



The process of tissue regeneration, whether in normal life or as 

 a reaction to injury, undoubtedly retards the aging of the tissue 

 or organ concerned as a whole, but the question whether it involves 

 an actual dedifferentiation and rejuvenescence of the cells concerned 

 in the regeneration must be briefly considered. Minot ('08, '13) 

 has attempted to prove that dedifferentiation does not actually 



293 



