298 WALDO SHUMWAY 



METHODS 



All the experiments described in this paper were carried on 

 upon a race of Paramaecium aurelia descended from an individual 

 isolated March 12, 1913 from a mass culture which had been main- 

 tained in the laboratory for several months. This race was 

 hereafter maintained in four files^ by daily isolations following the 

 method described by Calkins ('02). The principal deviation 

 was in the use of solid shallow watch glasses instead of depres- 

 sion slides. A daily record was kept of all the divisions which 

 occurred in the history of the race, and these were averaged at the 

 end of every five days to give the average division rate per day of 

 the race for that period. 



At the beginning of the experiments, two or three sister cells 

 were isolated from each file, of which one was used to continue 

 the control file while the others were used to start experimental 

 files. In this way two or more lines of four files each were ob- 

 tained of approximately the same protoplasm. Diagram 1 

 illustrates this method graphically. Wliile one of the lines thus 

 obtained was continued as the control line, the others were 

 treated with thyroid or other substances as hereafter described. 

 At the end of every five-day period, the division rates of the 

 experimental lines were ascertained and compared with that of the 

 control line for the same period. 



The accompanying graph (diagram 2) shows the history of the 

 control line (A) by five-day periods from the beginning of the 

 experiment until December 20, 1913, when the line died out. 

 The history of several of the experimental lines is indicated by 

 dots and dashes. 



The medium upon which the control line was kept was equiva- 

 lent to one-half ''standard hay infusion" (Jennings '10). It was 

 prepared by boiling one gram of hay in one hundred cubic centi- 

 metres of tap water for ten minutes, filtering and allowing it to 

 stand exposed to the air for twenty-four hours, when it was mixed 



1 The term "file" is used to designate a single genetic series of Paramaecia 

 derived from an original individual by the process of daily isolating one of the 

 cells produced by the divisions of the preceding isolated cell. 



