300 SENESCENCE AND REJUVENESCENCE 



occupy a position intermediate between the higher vertebrates 

 and the lower invertebrates, such as Planaria. Thus far Mr. Wells 

 has found that the susceptibility to cyanide and lack of oxygen 

 decreases early in starvation and remains more or less constant 

 during the first month or six weeks and then undergoes a rapid 

 increase and may become as high as that of well-fed growing indi- 

 viduals of much smaller size than the starved animals at the begin- 

 ning of the experiment. Apparently the rate of metabolism falls 

 early in starvation and remains relatively low for several weeks while 

 decrease in weight goes on, but after several weeks the rate begins to 

 rise and may reach that of animals which are physiologically much 

 younger than the starved animals were at the beginning. During 

 the first part of the period the starved fishes behave as regards rate 

 of metabolism like the warm-blooded animals, but later a rise in 

 rate occurs like that which the planarians show from the beginning. 

 Any attempt at interpretation of these results must at present, 

 however, be little more than a guess. The experiments suggest 

 that after removal or transformation of certain constituents of the 

 substratum the cells begin to burn themselves up at an increasingly 

 rapid rate as in Planaria, and so a much greater degree of rejuvenes- 

 cence occurs, at least in some tissues, than in the mammals. 



It has long been known that frogs and salamanders may live 

 for long periods of time without food and may undergo a consider- 

 able degree of reduction during starvation. In his studies of the 

 effects of starvation on members of this group Morgulis ('ii, '13) 

 has found that protracted starvation has a distinctly rejuvenating 

 effect. After starvation the animals grow more rapidly, use a 

 larger percentage of the nutrition in growth, and attain larger size 

 than those continuously fed. Contrary to von Seeland (p. 298), 

 Morgulis finds that intermittent starvation has a stunting effect, 

 but suggests that in his experiments the animals did not com- 

 pletely recover between starvation periods. 



In man and the higher vertebrates and probably also in the 

 higher invertebrates, such as the insects, individuation and differ- 

 entiation have progressed so far that after the earlier stages of 

 development any considerable degree of reduction or regression is 

 impossible under ordinary conditions without endangering in one 



