342 THE AMERICAN NATURALIST [Vol. LV 



Figure 19 illustrates the fact that the size of the entire 

 specimens increases during nuclear multiplication and 

 growth, but that the area per nucleus remains almost 

 constant. 



7. The series of measurements of these specimens af- 

 fords an explanation of the reason why nuclear division 

 in Opalina is not s^Tichronous. According to the nucleo- 

 cytoplasmic relation theory, as soon as the mass of cyto- 

 plasm has increased to a certain point nuclear division 

 is initiated. The necessary increase to furnish this 

 stimulus in Opalina may be determined approximately 

 from our data by comparing measurements of specimens 

 in which the nuclei are all equal in size with those in 

 which nuclear division has been inaugurated. Such a 

 comparison gives the following results. 



