FERTILIZATION 211 



through conjugation, the organisms may in some cases recover 

 their original vigor and begin a new cycle with youth renewed. 



But rejuvenation is by no means always the result of conjuga- 

 tion, for frequently the senescent organisms perish in spite of 

 conjugation; and it may even be the case that the descendants 

 of cells which have conjugated before the signs of senescence 

 have appeared, perish sooner than their immediate relatives 

 of approximately the same age, which have been prevented from 

 conjugating. Moreover, Jennings has shown that in certain 

 races of Paramoecium aurelia-caudatum conjugation may occur 

 at intervals of only one or two weeks, while in other races of 

 the same species conjugation occurs only at intervals of a year 

 or longer, and in still a third race no conjugation was observed 

 during a period of three years, although during this time 

 observation was not so continuous as to preclude the possibility 

 of conjugation having occurred. 



Valuable evidence upon the question of the cyclic character 

 of the Protozoan life history is afforded by the work of Woodruff, 

 who has shown that if more natural conditions are substituted 

 for the artificial and more uniform conditions of the laboratory, 

 no cyclic relation appears, in some strains of Paramcecium at 

 least. By continually altering the character of the food, and 

 by imitating in other ways the naturally variable conditions 

 of pond life, he has been able to continue a single race of 

 Paramoecium for over five years. During this period more 

 than 3000 generations were formed by simple fission, and in 

 all this time conjugation did not occur, and no periods of 

 depression or signs of structural modification could be ob- 

 served. 1 Finally, Woodruff has been able to carry a culture of 

 Paramoecium on a uniform diet of beef extract, which is sup- 

 posed to contain all of the materials necessary for their life, 

 for ten months (about 450 generations) without any indication 

 of senescence. 



Such facts as the foregoing show, first, that protoplasmic 

 activity among the Ciliates may not be cyclic in character under 



1 On Sept. 27th, 1912, Professor Woodruff writes that this culture is 

 in its 3265th generation, and still normal. 



