298 ELEMENTS OF BIOLOGY 



animal life, but this high degree of differentiation has been at the 

 expense of ability to grow continuously and to regenerate lost parts. 

 In animals which may reproduce agamically a physiologically young 

 condition is restored to the products of the reproductive division; re- 

 juvenescence by this method is denied the mammals, lost in the 

 high differentiation and specialization of the tissues. Individual 

 mammals grow old; only the mammalian fertilized eggs become 

 young again. 



Rejuvenescence. When animals grow old their metabolic 

 processes slow down, the denser products of metabolism accumulate 

 in the cells and intercellular substance, and anabolism, the construc- 

 tive phase of metabolism, becomes less and less conspicuous, while 

 the katabolic phase becomes more prominent. Eventually as a result 

 of this shift from a predominantly constructive to a destructive 

 metabolism, the protoplasmic structure is altered, energy transfor- 

 mation decreases below that necessary for continued maintenance, 

 and a disorganization familiarly called death ensues. 



From the beginnings of human intelligence men have sought to 

 reverse the process of growing old and to attain immortality of the 

 human body. For "the most familiar imperfection of Mankind is 

 growing old." We have seen that chemical reactions constitute the 

 important part of metabolism and that growth, differentiation, and 

 senescence are some of the consequences of metabolism. According 

 to commonly accepted chemical theory, all chemical reactions are 

 reversible. Are growth, differentiation, and senescence chemical 

 processes that are reversible in the human body? If so, rejuve- 

 nescence and restoration of youth to an old person should be 

 possible. 



In some of the lower invertebrates it is possible to demonstrate 

 the reversibility of these processes. If a well-fed large flatworm is 

 starved it becomes smaller and smaller. A planarian three-quarters 

 of an inch long may be reduced by several months' starvation to 

 an animal approximately one-eighth of an inch in length and hav- 



