CONDITIONS OF GAMETE FORMATION 381 



If senescence is essentially a decrease in rate of metabolism, the 

 stage of maturity must possess a lower rate of metabolism than the 

 stage in which agamic reproduction is occurring. While I have not 

 as yet made a systematic study of the changes in susceptibility of 

 ciliates with changes in medium and other conditions, certain 

 differences in susceptibility observed in a culture of Colpidium are 

 of some interest in this connection. This culture was at first under- 

 going very rapid agamic reproduction and the small, recently 

 divided individuals, as well as most of those in late stages of division, 

 were more susceptible to cyanide than the larger, older individuals. 

 In the course of a few days an acute epidemic of conjugation 

 occurred in the culture, and fissions almost ceased: conjugation 

 was confined to the larger individuals of the culture. At this stage 

 the small animals were most susceptible, the large, non-conjugating 

 animals less susceptible, and the conjugating pairs least susceptible 

 of all. The low susceptibility of the conjugants indicates that they 

 possess a lower metabolic rate and so are physiologically older than 

 the other members of the culture. 



These experiments suggest that the occurrence of conjugation 

 is associated with the attainment of a certain physiological age, a 

 condition of maturity, with a relatively low rate of metabolism. 

 If this conclusion is correct, we must consider the question of the 

 influence of external conditions upon the attainment of this ma- 

 turity: is it possible to accelerate or retard its occurrence through 

 cultural or other environmental conditions? It is not to be 

 expected that a sudden decrease in rate of metabolism induced by 

 external conditions will bring about a normal maturity in a very 

 young individual : such a change would simply retard or inhibit its 

 development. But when development has reached a certain stage 

 and the organism is approaching maturity, then it is very probable 

 that a slight decrease in metabolic rate, externally induced, may be 

 sufficient in many cases to bring about or accelerate the change 

 which under constant external conditions would have occurred much 

 more slowly. Some of the chemical agents which Zweibaum and 

 others have used to induce conjugation may perhaps act in this way. 



As regards the different capacity or tendency of different races 

 to conjugate, which has been discussed by Jennings, Woodruff, and 



