6oo Lorande Loss Woodruff. 



for this purpose exactness was most essential.^ Whatever factor or 

 factors caused the high division-rate of the fifth period, the effect 

 was not lasting for in all of the succeeding periods up to, and 

 including, the thirteenth, the rate steadily decreased and at a 

 remarkably uniform rate. During the twelfth period (end of Sep- 

 tember) I moved the culture back to Williamstown. No effect is to 

 be seen in the succeeding period but the rise in the fourteenth 

 period undoubtedly is due to this change. This time the rise 

 was by no means so marked and it was evident only after a latent 

 period of ten days or more. This possibly can be explained by 

 the fact that the "potential of vitality" of the infusorians was 

 considerably less than when the first removal took place.* Like 

 the acceleration at the first removal, this second one was not 

 lasting as, during the following ten-day period, the fission-rate 

 settled down to where we should expect to find it if the culture 

 had been carried along without any disturbing influence. Begin- 

 ning with the next period (No. i6) the very exact treatment which 

 I had employed was discontinued and the change of liquid was 

 made only every other day, and then not at exactly forty-eight 

 hour intervals. The effect of this is at once apparent in the con- 

 siderable fluctuations in the fission-rate shown in the culture- 

 curve during the remaining four periods of the life of the culture. 

 At the beginning of period 20, i. e., at the 191st day of the life of 

 the series, when the animals were dividing on the average three 

 times in two days, the culture suddenly died out, stock and all, 

 at the 288th generation. I noted that the infusorians were 

 exceptionally active on the slides just previous to their extinction. 

 This sudden death of the culture cannot be attributed to any 

 accidental change in the liquid medium as the stock was affected 

 similarly at the same time.^ 



ij endeavored to secure this uniformity of treatment and culture medium: (i) By changing the 

 culture medium daily and at the same hour, thus making the daily records of just twenty-four-hour 

 periods. (2) By using the same kind of grass and grass grown in the same place. (3) By washing the 

 grass very thoroughly and boiling it for one minute. This was given as soon as it reached the room 

 temperatuie. 



^Calkins ('02, i) found, however, that a Journey which he made with his Paramoecium cultures when 

 they were on a descending cycle accelerated the fission-rate, while a return journey made when the cul- 

 tures were on the ascending cycle produced a retarding effect. 



'It will be recalled that the death of Maupas's culture of Stylonychia pustulata was preceded by 

 a period of more rapid division of almost' three weeks' duration. 



