460 GARY N. CALKINS 



spectively, per ten days during the same calendar period. It 

 lived for 135 division days and for 192 divisions and had a rela- 

 tive vitality of 51.5 per cent. 



The cut fragment giving rise to the VX2 series required eight 

 days for complete reorganization and divided for the first time 

 on the ninth day after the operation (fig. 8) . The five lines of the 

 series were established on October 26th, and during the first 

 sixty days each divided at the rate of 15 times per ten days, the 

 control Z series and the parent PV series dividing as stated above. 

 This experimental series lived for 101 division days, during which 

 it passed through 120 generations. The relative vitality was the 

 lowest of all the experimental series, being only 31.8 per cent. 



The VX3 series was derived from an individual cut during a 

 latie stage of conjugation after completion of the maturation proc- 

 esses and after nuclear interchange (figs. 7 and 8). The five 

 lines were established on October 26th, and the average division 

 rate for the first sixty days was 16 times per ten days, while the 

 normal ex-con jugant control and the parent PV series divided at 

 the rates of 15.7 and 3.8 times per ten days during the same 

 period. This experimental series lived for 140 division days 

 and 202 generations and its relative vitality was 51.3 per cent. 



The XZ2 series, finally, was derived from an -individual which 

 had just completed conjugation and had begun to reorganize. 

 The anterior end, including the peristome, was removed by 

 cutting. Reorganization was completed in five days, and the 

 five lines were established on the seventh day after cutting 

 (fig. 8). The division rate per ten days for the first sixty days 

 was 15.6 divisions, or the same as that of the normal ex-conju- 

 gant control Z series (15.7). This experimental series lived 

 for 163 division days and for 202 generations and had a relative 

 vitality of 52.2 per cent. 



For purposes of reference and comparison, the above experi- 

 mental results are summarized in tabular form in table 1 . 



The curve of the life-cycle of each of these experimental series 

 agrees with the typical normal curve as plotted for successive 

 ten-day periods. The initial division rate is high (first sixty 

 days) and gradually decreases to nothing at the end of the life- 

 history. 



