346 THE- AMERICAN NATURALIST [Vol. LV 



nuclei becomes greater with advancing age. Since the 

 volume of the nuclei is less in these older specimens the 

 amount of cytoplasm associated in normal nucleo- 

 cytoplasmic relations with them is less and the area of 

 the specimens per nucleus decreases accordingly. 



The curves in Fig. 20 bring out clearly the relation be- 

 tween area and volume and surface of the nuclei during 

 the growth period. The average area per nucleus de- 

 creases as the number of nuclei increases, but at the same 

 time there is a corresponding decrease in both volume 

 and surface of the nuclei, thus maintaining approximately 

 the initial relation between nucleus and cytoplasm. 



Summary 



(a) A high correlation exists between nuclear number 

 and cytoplasmic mass (as indicated by area) during the 

 growth of Opalina sp. The coefficient of correlation in 

 one lot of 341 specimens was .755 ± .016 and in another 

 lot of 144 specimens was .874 ± .015. 



(b) By comparing the area of various stages with the 

 number, size, state of division, volume and surface of the 

 nuclei the following conclusions were reached. (1) Nu- 

 clear division is stimulated by an increase of cytoplasm 

 that may be determined approximately. (2) As the 

 organisms increase in age the nuclei decrease in volume 

 and surface ; this is accompanied by a corresponding de- 

 crease in the area per nucleus, indicating that the nuclec- 

 cytoplasmic relation is maintained. (3) Nuclear division 

 is not s\Tichronous because one nucleus is usually stimu- 

 lated to divide before the others, and this division is suffi- 

 cient for the time to reestablish the normal relation be- 

 tween nuclei and cytoplasm. 



