326 H. S. JENNINGS 



ular stock of A (fig. 1) those cultivated on slides became very 

 unhealthy in June, so that all died out June 18; the branch was 

 preserved only because some of them had thus been removed to 

 watch glasses on June 7, forming our present stock D (fig. 2). 



Thus all the evidence, from many distinct sources, points to 

 the explanation we have set forth above, that the non-conjugants 

 of the slide series D are less vigorous than either the non-conju- 

 gants (H) or the conjugants (K) of the watch glass series, because 

 they were cultivated longer on slides. 



Experiment 12: Conjugation in a depressed stock: August 10 to 

 September 7: Paramecium aurelia 



Whatever the cause, we now have on hand, after the Experi- 

 ments 11 a to 11 ^, a much depressed stock, which has omitted 

 at least four normal conjugations. Now, it might be maintained 

 that our uniformly negative results thus far as to rejuvenescence 

 by conjugation are due to the fact that we were not dealing with 

 depressed stocks. It is of interest, therefore, to determine the 

 effects of conjugation within a stock thus known to be depressed. 

 There is, however, great difficulty in carrying out such an experi- 

 ment, for such depressed stocks cannot easily be induced to 

 conjugate. The main condition for conjugation appears to be, 

 that there shall be a period of rapid multiplication, followed by a 

 decline in the conditions inducing it. But in such depressed 

 stocks it is almost impossible to induce rapid multiplication. 

 After many efforts, I finally succeeded on August 10 in getting 

 a scanty conjugation in a watch glass culture of this depressed 

 set of the slide series of D (fig. 2) which has descended without 

 previous conjugation from the split pair of March 4. I was able 

 to obtain for study but three pairs, the ex-conjugants of course 

 forming when isolated six lines of propagation. I call these 

 sets D 1. 



From the same watch glass culture I isolated at the same time 

 ten of the individuals that were not conjugating, and from these, 

 ten lines of propagation were derived, which were kept under the 

 same conditions' as those from the conjugants. These may be 

 called D 2. 



