ARTICLES 



619 



With these data in mind, a plausible hypothesis imme- 

 diately suggests itself to account for the differences in size 

 and spine number between different strains of these organisms. 

 This hypothesis is that the quantity of chromatin within the 

 nucleus determines the limits of the reactions of the nucleus 

 with the cytoplasm, and that the greater the chromatin mass, 

 the greater the quantity of cytoplasm associated with it, and 

 the larger the specimen will be that contains it. Since the 

 chromatin mass can be measured, and its size determined, 

 this hypothesis could be tested. It was so tested and found 

 to be correct. For example, the chromatin masses of specimens 



Fig. 9. 



belonging to a small family were measured, and a corresponding 

 number from a large family, with the following results : 



Small Family 

 Large Family 



No. of 

 Specimens 

 measured. 



40 

 40 



Average 



Diameter 



in Microns. 



112 

 I50 



Average Spine 

 Number. 



9-91 

 14*25 



Comparative 



Diameter of 



Chromatin Mass 



in Units. 



35' 66 



47-82 



As the hypothesis demands, the quantity of chromatin 

 within the nuclei of the small family is less than that within 

 the nuclei of the large family. The differences in size among 

 the families reared from single " wild " specimens appear, 

 therefore, to be due to differences in the quantity of chromatin 

 present in them. Similarly, the difference in size and spine 

 number between the two branches of the large family described 

 above are probably also due to a diversity in chromatin mass, 

 although the origin of this diversity is problematical. 



Arcella polypora. — The characters of this species of Arcella 

 that were studied are diameter, nuclear number, and chromatin 

 mass. As in Arcella dentata, families of this species reared 

 from single " wild " specimens by vegetative reproduction 



