UROLEPTUS MOBILIS ENGELM. 457 



gave rise to two filial series which showed characteristic renewal 

 of vitality and passed through normal, but weak, life-cycles. 

 This weakness, indicated by the low percentages of relative 

 vitality, may have been due to inheritance or to the old age of 

 the parent at the time of conjugation (see Uroleptus III, loc. 

 cit., for effect of parents' age on vitality of offspring). 



A second group of cutting experiments was undertaken with 

 conjugating pairs from this PX7 series. One series only", the 

 X75 series, was successful. This pair was cut on October 10, 

 1919, when the parent series was in the 203rd generation. Conju- 

 gation had progressed to the stage of pronuclei migration, but 

 the two nuclei were removed with the apex of the V by the 

 operation (fig. 5). The experimental individual thus had com- 

 pleted its maturation processes, but fertilization had been pre- 

 vented by removal of the migrating pronucleus (fig. 6). Six 

 days were required for its reorganization, and it divided for the 

 first time on October 18th. During its first sixty days of culture 

 it divided on the average 15.5 times per ten days, while the parent 

 series during the same period divided only 5.4 times. This 

 series lived for only 123 division days and divided only 181 

 times, hence its relative vitality was low, only 46.3 per cent. 



Material for a third group of cuttings was obtained from the PV 

 series, which had served as the normal ex-conjugant control for 

 the first group of cutting experiments. At the time of the 

 experiments, October 15, 1919, the PV series was in its 270th 

 generation. It had had a vigorous ancestry and its own relative 

 vitality was high (88.1 per cent), but it was well advanced in 

 age when the conjugation epidemic supplying the material for 

 •this group of cutting experiments occurred. A low relative 

 vitality of the progeny was therefore to be expected, and this 

 expectation was reahzed by the life-cycle of the normal ex- 

 conjugant control for the experimental series, the Z series. This 

 control series divided at the rate of 15.7 times in ten days for the 

 first sixty days, while the parent PV series during the same 

 calendar period divided at the rate of 3.8 times per ten days. 

 The Z series lived for 168 division days, during which it divided 

 226 times and had a relative vitality of 57.8 per cent. 



