152 GAEY N. CALKINS 



genesis is as great as, or even greater than, that after conjuga- 

 tion. It remains to be seen whether this high potential has the 

 same capacity of endurance as that obtained from conjugation. 

 The M series is not old enough at the present time to furnish 

 evidence. The B series divided only 258 times, while its con- 

 temporaries, the A and C series, divided 313 and 348 times, 

 respectively. 



GENERAL 



It is not my intention to formulate here any theory in explana- 

 tion of the phenomenon of rejuvenescence. The facts con- 

 cerning it may be grouped in two categories: one, physiological, 

 the other, morphological. 



In the first place, the results presented in this paper show that 

 in Uroleptus mobilis the physiological processes of metabolism 

 are not capable of unlimited activity. The limits vary from 

 the time of conjugation or encystment to between 268 (H series) 

 and 349 (C series) generations by division. Within these limits 

 there is a progressive weakening of metabolic vigor from an opti- 

 mum shown during the first three months after conjugation. 

 This weakening, furthermore, increases by geometrical progres- 

 sion, i.e., it is cumulative, as shown by the geometrical increase 

 of the difference in vitality between filial and parental series in 

 successive sixty-day periods (table 5). Such weakening proto- 

 plasm, if not allowed to conjugate, inevitably dies, as does the 

 somatic protoplasm of metazoa. 



A second physiological fact is equally well established by these 

 Uroleptus experiments. The same protoplasm is transformed 

 from a condition of metabolic weakness to a condition of opti- 

 mum metabolic vigor by the . process of conjugation. The effect 

 of conjugation is clearly indicated by the extreme case of the J 

 series. Here the protoplasm, at the time of conjugation, had 

 only enough metabolic vigor to divide twice in ten days. The 

 cells that conjugated were both composed of this weak proto- 

 plasm. Ten days later if they had not conjugated, each might 

 have been able to divide at the rate of 0.25 times in ten days, 

 or once in forty days; but, having conjugated, one of them, the 



