516 G. N. CALKINS AND L. H. GREGORY 



.and a fourth a little later; so that two-thirds of the conjugant 

 lines were dead. But two continued to multiply. But in the 

 meantime all of the ten non-conjugant lines died out" (p. 375). 

 Here then, was a race in depression; they were allowed to con- 

 jugate and did conjugate after repeated efforts to make them do 

 so. Two of the six lines that had conjugated continued to live, 

 the other four lines died. Jennings says: ''We can hardl}^ speak 

 of rejuvenescence where two-thirds of the ex-conjugants die out. 

 The survival of some of the conjugants may have been due to 

 the greater vigor that was a pre-requisite to their conjugation, 

 the lack of which caused the others not to conjugate. Aside 

 from this we can only say that the results of conjugation were 

 here the same as usual; it induced variation in the reproductive 

 power" (id., p. 275). 



Now it seems to us that this is a very clear case of rejuvenes- 

 cence. Mortality of the ex-conjugants, amounting to 66 per cent 

 was not as great as the mortahty found by us in the history of 

 360 ex-conjugants where it was 82 per cent. Until we know 

 what causes this high mortality, or the mortality of 100 per cent 

 in Blepharisma and in Stylonychia (Baitsell '12) after conjuga- 

 tion we cannoft use mortality as a test of rejuvenescence. The 

 question of vigor which Jennings raises in those able to conjugate, 

 is purely hypothetical; it might have been tested by the method 

 of split conjugations, but this was not tried. Had this been done 

 we believe the lines derived from the split pairs would not have 

 multiplied any more vigorously than the ten control lines which 

 did die out. This experiment therefore appears to be positive 

 evidence of rejuvenescence through conjugation. 



Jennings makes the statement: ''So far as I have been able to 

 discover, there is no experimental evidence from any orher source 

 (viz., that given by Maupas) that conjugation rejuvenates" ('13, 

 p. 374). We would call attention here to one very clearly de- 

 fined experiment showing successful rejuvenescence after con- 

 jugation which was described in Calkins' paper of 1904. Since 

 this seems to have been overlooked by Jennings, and because of 

 its unique character, the description may be repeated here with 

 somewhat fuller details. 



