468 G. N. CALKINS AND L. H. GREGORY 



8. Discussion 504 



A. The potential of vitality 504 



B. Are all Paramecia potential germ cells 507 



C; Do some Protozoa die a 'natural death' 509 



D. Variations in Paramecium 511 



E. Rejuvenation or variability 514 



9. Summary 521 



10. Bibliography 524 



1. NATURE OF THE PROBLEM 



The traditional view, advocated by Weismann, 1881, that all 

 protozoa are potential germ cells, has held the stage for so long 

 that it seems almost iconoclastic to doubt the statement. Re- 

 cent writings, however, seem to foreshadow a change of opinion. 

 Woodruff writes('09): ''I believe these results indicate that the 

 phenomenon (i.e., conjugation) is not so frequent in the life his- 

 tory as is generally believed" (p. 304). Nor was Woodruff able 

 to induce the individuals in his long culture to conjugate: ''There 

 has been no tendency to conjugate among the paramecia of this 

 culture and a large series of experiments, which have been made 

 with the individuals left over from the four lines of the culture 

 after the daily isolations, have, so far, yielded not a single pair of 

 conjugants" (1911, p. 264). In striking contrast to this result 

 is that obtained bj^ Jennings with a race of the same species as 

 that used by Woodruff (Param. aurelia). The individuals con- 

 jugate every two or three weeks and can be depended upon to 

 conjugate whenever the conditions are provided (race k). But 

 along with this conjugating race Jennings has cultivated other 

 strains of P. which have never been observed to conjugate (e.g., 

 race D, Jennings '10), while other races possessed the power in 

 varying degrees. Furthermore, it is a matter of common expe- 

 rience to find wild cultures in the laboratory which yield epidem- 

 ics of conjugation while other cultures equally rich give none at 

 all. 



The results of previous careful experimental work therefore 

 shows that some races of Paramecium will conjugate, some will 

 not, under the conditions usually employed to induce conjuga- 



