38 R. T. YOUNG 



depressing agents, regardless of their character, will affect the 

 nuclear condition of Paramaecium. 



In these experiments the animals were kept at room tem- 

 perature, varying from 13° to 25°C. 2 For the most part the room 

 was partly lighted during the day from a west window and at 

 night was dark. The nuclear condition at each isolation with 

 few exceptions was determined by staining with methyl green. 

 In a few cases permanent mounts stained in haematoxylin were 

 made. This was not done daily, however, because of the labor 

 involved, and because I was only interested in the gross condi- 

 tion of the macronucleus, whether single or multiple. The de- 

 tails of the nuclear changes in endomixis have been worked out 

 by Woodruff and Erdmann and do not concern us here. 



The frequency of endomixis I have determined by the number 

 of generations between two of them, rather than by the time 

 interval, as the former indicates better than the latter the 

 amount of growth between the periods. 



Experiment 1 (figs. I and la) 



In this experiment line I was carried for one-hundred and eighty 

 (two) generations 3 on the same slide, with occasional changes to a 

 fresh slide. The influence of a change of slide with consequent nearly 

 complete change of culture was usually shown in the marked increase 

 of division rate of the animals. Occasionally, however, such an increase 

 appeared when no apparent external cause' was responsible. Such in- 

 creases moreover were not invariably associated with endomixis. After 

 one hundred forty-seven (eight) generations line la was branched off 

 and carried with fresh slide changes daily from 5/8 to 6/19. The aver- 

 age daily division rate, during the first part of this experiment, when 

 the slide was changed only occasionally, was 1.4; while during the latter 



2 In experiment 2, line II (2/22-4/15, 4/17-5/2) the slide was carried in a 

 thermostat, ranging between 19° and 22° C. and in experiments 2, 4, 5, 6, and 13, 

 lines Ha, IV, V, VI and XIII. (5/23-6/19) at 20°-23°C. as a check on those 

 conducted at room temperature, but without noting any difference in the re- 

 sults. The range of 12°C. to which the animals were subjected at room tem- 

 perature more nearly simulates natural conditions, than does that of only 2°-3° 

 maintained in a thermostat. The extremes of 13° and 25° in room temperature 

 were experienced but rarely and for short intervals. The thermostat lines neces- 

 sarily experienced a slight change of temperature daily during periods of 

 examination. 



3 Uncertainty regarding the counts in two cases renders the total number of 

 generations uncertain to the extent of two. The point is immaterial however. ■ 



