The Life History of Tillina Magna 421 



100// by 50/<. These facts seem to prove that there is no relation 

 between the amount of nuclear material in the cell, and the general 

 vitality of the protoplasm. In other words, the periods of weak- 

 ness are not caused by an excess of nuclear material. The nucleus 

 may or may not increase in size during periods of low activity; 

 if an increase does take place, it is generally found that the cyto- 

 plasmic material has increased also, and the ratio between the 

 two is the same as in the periods of high activity. Formerly it was 

 thought that as age advances the cells diminish in size. Wood- 

 ruff, however, found that there is no diminution in size until just 

 before death, the shrinkage then being normal, since the metabolic 

 functions had practically ceased. I have found a similar condi- 

 tion in the protoplasm of Tillina. The size averages of nucleus 

 and cytoplasm are practically the same throughout the life history. 



Finally, in order to make my points clear, I will summarize the 

 chief differences between the results of Popoff and my own. 



If the curve of Stylonychia made from Popoff's data is plotted 

 in the same manner as that of Tillina, it will be found that four 

 out of the five so-called depression periods resolve themselves 

 into normal rhythmic fluctuations, from which recovery takes 

 place without external influence, and Hertwig and Popoff are quite 

 wrong in considering them "depression periods" in Calkins' 

 sense. The fifth period which marks the end of the cycle may or 

 may not have been a true period of depression, as all died, not 

 being stimulated in any way. 



Actual measurements of Tillina, show that during normal fluc- 

 tuating periods, the so-called "depression periods" of Popoff", as 

 well as in the actual periods of weakness, there is but little and no 

 regular change in the size relations of nucleus and cytoplasm. 



The curve plotted from the resultants obtained by averaging 

 the ratios of cell area to nuclear area, shows no definite relation 

 to the curve of vitality as would be expected on the theory of Hert- 

 wig and Popoff. 



The size of Tillina has been found to be practically the same 

 at the end of the life history as at the beginning. 



The work of Maupas and Calkins has shown that conjugation 

 does not take place during the depression periods, but prior to 



