30 



GENERAL CONSIDERATIONS 



generations is kept for all lines. The total number of divisions is 

 recorded for all lines and these are averaged for ten-day periods. The 

 division rate is taken as a measure of vitality and the history of the 

 protoplasm is shown graphically by plotting the ten-day averages on 

 a graph in which the ordinates represent the number of divisions and 

 the abscissas the consecutive ten-day periods. Such a graph for U. niobilis 

 is shown in Figure 10, which is a composite graph of 23 different series, 



Figure 10. Uroleptus mobilis Engelm. Graph representing the life history by ten- 

 day periods. (After Calkins, 1933.) 



averaged for successive ten-day periods in isolation culture. It will be seen 

 that there is a high initial vitality which gradually wanes through 360 

 divisions during 300 days. In these isolation cultures the individuals 

 do not encyst nor conjugate; hence division is the only means of re- 

 organization. 



That division is not effective in checking waning vitality is shown 

 by the fact that, in my experience, all such cultures of Uroleptus finally 

 die, in some cases in from fourteen to sixteen months, in other cases 

 in from three to ten months. 



The macronucleus is probably the most important of all derived 

 organs of Uroleptus. With each division of the cell, as shown above, it 

 undergoes a process of reorganization whereby the chromatin is restored 



