558 



BIOLOGY OF THE PROTOZOA 



of change in natnre of rejuvenescence in later series; (5) exhaustion 

 of a strain through continued old-age breeding; (6) strengthening 

 a weakened strain by early conjugations. 



1. Renewal of Vitality as a Result of Conjugation —In Chapter X 

 it was shown that the life cycle of an ex-conjugant of Vroleptus 

 mobili.s' begins with high vitality; this gradually weakens during 

 a period of from nine to twehe months and ends with death of the 

 last individual representing that protoplasm if reorganization by 

 fertilization or parthenogenesis has been prevented. A full pedigree 

 of the latest series (128) is illustrated l)y the graph shown in Fig. 



Fig. 232. — Condensed vitality graphs showing the descent of Uroleptus mohilis from 

 November, 1917 to date. S=series; G = generation age of parents. 



232. Conjugation between the progeny of an ex-conjugant occurs 

 whenever a conjugation test is made after the series is mature 

 (see p. 490). An ex-conjugant from such a mating has a higher 

 vitality as expressed by the division-rate than the individuals 

 of the parent series which had not conjugated. The test for this 

 is shown by a comparison of the division-rate of the parent proto- 

 plasm which has not conjugated with the diA^ision-rate of the 

 protoplasm that had conjugated, both protoplasms running simul- 

 taneously and under identical conditions in isolation cultures. 

 If such conjugations occur early in the life history of the parent 

 series both parent and offspring run simultaneously for some months; 



