STUDIES ON LIFE-HISTORY OF PROTOZOA. 195 



dition of the general vitality at different periods by this means. 

 Good results were finally obtained by averaging the number of 

 divisions per day for definite periods, and plotting the averages. 

 By this method of illustration the condition of the general vitality 

 at any period desired can be seen at a glance. (See Fig. i.) 



The division-rate, as represented by such a curve, continued 

 without noticeable variations from the beginning of the experiments 

 until the latter part of the following July. About the 2Oth of that 

 month the organisms began to die off at an alarming rate, indicating 

 that a period of depression had apparently set in, or "degenera- 

 tion " in the Maupas sense. Now was the time, if at all, to ex- 

 periment with substitutes for conjugation, and in conformity with 

 my original plan, the first trials were made with a change of diet. 

 The organisms had been living steadily on the bacteria that de- 

 velop in hay-infusion (Bacillus snbtilis mainly), and various sub- 

 stitutes were now tried. Vegetable infusions of different kinds 

 gave no improvement but meat infusions proved successful. The 

 first experiment with the latter was with teazed liver which was 

 added to the usual hay-infusion. The result was very gratifying 

 for the organisms began immediately to grow and to divide, the 

 rate of division rising from 5 to 9 divisions in successive lO-day 

 periods. The effect of the liver was not lasting, however, and in 

 the following lo-day period, the rate fell off to one division in 

 five days or two-tenths of a division per day. During this time 

 the organisms had not been continuously on liver treatment but 

 had been transferred to the usual hay-infusion diet after 48 hours 

 in the liver medium. At the end of the first lO-day period in 

 August the cultures were again dying off, this time more rapidly 

 than before and the renewed use of teazed liver had absolutely 

 no effect. One thing after another was now tried without suc- 

 cess, the organisms meanwhile growing weaker and weaker. 

 Finally they were transferred to the clear extract of lean beef 

 made by boiling fine pieces of beef in tap water. The effect of 

 this medium was interesting, for, although it restored the weakened 

 vitality, there was no rapid increase in the rate of division as when 

 first treated with the teazed liver. The infusoria were, however, 

 now large and vigorous and did not die unless transferred from the 

 beef medium to the usual hay infusion. When this was attempted, 



