122 



Scientific Proceedings (49). 



manding position, though it is neither contradictory nor con- 

 firmatory of the view that fertilization, resulting in amphimixis, 

 is concerned in some way with the phenomenon of variation, or 

 that it may be a process which enables certain forms to withstand 

 changed environmental conditions. 



The present paper briefly outlines the results which have been 

 obtained to date from an intensive study of a pedigreed race of 

 Paramcecium aurelia with reference to the problem of protoplasmic 

 senescence and the function of conjugation. I have previously 

 published 1 the results obtained to September, 1910, and the reader 

 is referred to earlier papers for further details of the culture and 

 for a general discussion of the various phases of the work. 



This culture was started on May 1, 1907, with a "wild" 

 ParamcBcium aurelia which was isolated from a laboratory aqua- 

 rium. This individual was placed in about five drops of culture 

 medium on a glass slide having a central ground concavity, and 

 when the animal by division had formed four individuals, each 

 of these was isolated on a separate slide to form the four lines, la, 

 lb, Ic, and Id, of this culture, Paramcecium I. The pedigreed 

 culture has been maintained by a specimen isolated from each 

 of these lines practically every day up to the present time, thus 

 precluding the possibility of conjugation occurring and facilitating 

 an accurate record of the number of generations attained. A 

 culture medium consisting of infusions of hay and fresh grass 

 was employed during the first nine months of the work, but there- 

 after infusions of nearly any materials which might be found in 

 ponds, swamps, etc., have been used. The medium has invariably 

 been boiled to render the introduction of "wild" individuals 

 into the culture absolutely impossible. 



This race of Paramcecium has attained so far, (May 1, 1912) 

 3,029 generations during the five years it has been under daily 

 observation. The number of generations attained during each 

 of the first five years of its existence is as follows: first year 452, 

 second year 690, third year 613, fourth year 612, and fifth year 

 662. The mean rate of division for the entire period is over three 

 divisions in forty-eight hours. Periods of marked physiological 

 depression have not occurred — such variations in the rate of 



1 Archiv fiir Protistenkunde, Bd. 21, 4. 



