THE CELLULAR BASIS 99 



ter grows indicates that every minute particle 

 of protoplasm has this power of taking in food 

 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 fifi (millionths of a millimeter) in 

 diameter. The largest molecules are probably 

 about 10 IJ.^1 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 different in different 

 species, or with respect to different hereditary 

 characters, they are known as inheritance imits. 

 It is assumed in practically all theories of 



