UROLEPTUS MOBILIS ENGELM. 453 



It was considered unnecessary to fix and stain one of the indi- 

 viduals of this pair because of the unmistakable evidence of recent 

 union. The cut end regenerated perfectly within eight hours 

 and the new macronucleus 'placenta' appeared in forty-eight 

 hours as a circular vesicle in the center of the cell (fig. 2). The 

 first division occurred on April 27th and the customary five 

 lines of the series were established on the 29th (see fig. 9 for 

 similar early history of any normal ex-conjugant). 



The parent P series was the fifth filial generation from the 

 original A series of Uroleptus mobilis, its full pedigree being: 

 A-C-F-I-L-P. All of these ancestral series, with one exception 

 (L) were characterized by high relative vitality as follows: A 

 90.9 per cent, C 97.7 per cent, F 94.1 per cent, I 89 per cent, 

 L 74.5 per cent, and P 95 per cent.^ So far as vitality was con- 

 cerned, therefore, the PXl series had an excellent heritage. 



This experimental series lived until February 14, 1920, the 

 last division occurring on February 7th, giving a life span of 

 286 division days, during which it divided 281 times. The 

 parent P series lived for 186 days after the PXl series was started 

 and divided 180 times, while the filial experimental series divided 

 190 times during these 186 days, indicating a well-marked 

 though slight rejuvenescence. The relative vitality of the 

 PXl series was much lower than that of the parent, being only 

 71.8 per cent as against 95 per cent. 



At the same time a normal ex-conjugant from the parent 

 P series was established in culture as a control series. This, the 

 PV series, lived for 265 division days and divided 317 times, with 

 a relative vitahty of 88.1 per cent, thus ranking in vigor with the 

 ancestral series. It divided on the average 15.5 times in ten 

 days during the first sixty days of life, while the parent series 

 during the same time divided 14.2 times. 



Two other pairs of conjugating Uroleptus from the same 

 parent P series were cut on the same day as the PXl series. These 

 were the PX6 series and the PX7 series. The former under- 

 went reorganization in seven days, while the permanent prepara- 

 tion shows that the individuals were in the stage of the second 



' See Studies III, Jour. Exp. ZooL, vol. 29, no. 2, p. 287. 



