MORE COMPLICATED LIFE CYCLES 131 



isms, or ratlier the race, were in the 200th generation at the time of the 

 first depression, but the vitahty of the protoplasm was not exhausted 

 until the 742d. Woodruff's race of oxytricha protoplasm was in the 

 235th generation at the first depression period, but lived through SCO 

 generations. There is no doubt whatsoever that all of the cells of 

 Paramecium would have died in the first period of depression had 

 nothing been done to revive them. Joukowsky, in 1898, followed 

 Paramecium through 170 generations, when they all died during a 

 period of depression; Simpson, in 1001, noted the gradual loss of 

 vitality and death in his three to four months' cultures of paramecium. 

 My cultures would have disappeared in a similar manner had it not 

 been for a change of diet, by which it was found that beef extract, if 

 given to paramecium for several days during this depression period, 

 would restore the vitalitv and start the organisms off on another cvcle 

 of cell generations. In this way the few surviving organisms of the 

 original culture were stimulated to new activity, or, to carry out the 

 analogy with the battery, were given a new potential of vitality and 

 a potential which again lasted through a period of six months, and 

 through approximately the same number of generations (actually, 198) 

 (see Fig. 38, period, August, 1901). 



How can the renewal be interpreted ? Obviously, the change in diet 

 gave the cells an entirely different assortment of chemical substances, 

 and it is to this fact that we may attribute the artificial rejuvenescence. 

 Woodruff found that the same expedient renewed the vitality of his 

 race of oxytricha, the effect being slower than in the case of para- 

 mecium. It was also found by Calkins that a change in the salt con- 

 tent of the usual food media would produce a similar stimulating effect, 

 and dilute solutions of potassium phosphate were used, the organisms 

 experimented with being allowed to swim in the solutions for half 

 an hour (a longer period l)eing followed by death in a few days). 

 This simple salt, like the beef extract, was enough to renew the vitality, 

 and the stimulus thus given was sufficient to enable the organisms to 

 live again in the same medium for another cycle of 193 generations. 



The effect of the change on the organism's structure is of interest, 

 and is represented by Fig. 39. The cell in a depressed condition is 

 shown on the left; a cell twenty-four hours after treatment is shown in 

 the centre, w^here a lighter area in the vicinity of the nucleus will be 

 noted, the ends meanwhile showing the same densely granular struc- 

 ture as that of the depressed condition, thus indicating that the organ- 

 ism is recovering from the disease, if we may so designate its trouble. 

 It is important, in this connection, to note that the reesta])lishing of 

 the normal structure occurs first in the neiojiljorhood of the nucleus, 

 a fact that indicates that here is the region of greatest chemical activity 

 m the cell. A cell forty-eight hours after successful stimulation is 

 shown on the nVht. These show that the "labile" condition of the 



