THE CELLULAR BASIS 105 



substance and of dividing into two particles 

 when it has grown to maximum size. Pre- 

 sumably this power of assimilation, growth and 

 division is possessed by particles of protoplasm 

 which are invisible with the highest powers of 

 our microscopes, though it is probable that 

 these particles are much larger than the 

 largest molecules known to chemistry. The 

 smallest particle which can be seen with the 

 most powerful microscope in ordinary light is 

 about 250 pp (millionths of a millimeter) in 

 diameter. The largest molecules are probably 

 about 10 [ifi in diameter. Between these in- 

 visible molecules and the just visible particles 

 of protoplasm there may be other units of or- 

 ganization. These hypothetical particles of 

 protoplasm have been supposed by many 

 authors to be the ultimate units of assimila- 

 tion, growth and division, and in so far as these 

 units are supposed to be the differential causes 

 of hereditary characters, they are known as in- 

 heritance units. 



It is assumed in practically all theories of 

 heredity that the "inheritance material," or 

 the germinal protoplasm, is composed of 



