1 08 THE POPULAR SCIENCE MONTHLY 



to generation, and whose characteristics, as compared with the general 

 protoplasm, are great stability, independence and continuity. This is 

 the idioplasm of Nageli, the germ-plasm of Weismann. But thtre is no 

 reason to suppose that "germ-plasm" is anything other than germinal 

 protoplasm, which is found in all cells in early stages of development 

 but which becomes limited in quantity or altered in quality in tissue 

 cells. A "germ-plasm" which is absolutely distinct from and inde- 

 pendent of the general protoplasm is a mere fiction which finds no 

 justification in reality. 



4. The Units of Living Matter 



The entire cell, nucleus and cytoplasm, is the ultimate unit of 

 living matter which is capable of independent existence. Neither the 

 nucleus nor the cytoplasm can for long live independently of each 

 other, but the entire cell can perform all the fundamental vital proc- 

 esses. It transforms food into its own living material, it grows and 

 divides, it is capable of responding to many kinds of stimuli. But 

 while the parts of a cell are not capable of independent existence they 

 may perform certain of these vital processes. 



Not only is the cell as a whole capable of assimilation, growth and 

 division, but every living part of the cell has this power. The nucleus 

 builds foreign substances into its own substance, and after it has grown 

 to a certain size it divides into two; the cytoplasm does the same, and 

 this process of assimilation, growth and division occurs in many parts 

 of the nucleus and cytoplasm, such as the chromosomes, chromomeres, 

 centrosomes, plastosomes, etc. In all cases cells come from cells, nuclei 

 from nuclei, chromosomes from chromosomes, centrosomes from centro- 

 somes, and probably plastosomes from plastosomes, etc. 



Indeed, the manner in which all living matter 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. Presumably 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 chemis- 

 try. The smallest particle which can be seen with the most powerful 

 microscope in ordinary light is about 250 pp (millionths of a milli- 

 meter) in diameter. The largest molecules are probably about 10 pp 

 in diameter. Between these invisible molecules and the just visible par- 

 ticles of protoplasm there may be other units of organization. These 

 hypothetical particles of protoplasm have been supposed by many 

 authors to be the ultimate units of assimilation, growth and division. 

 In so far as these units are supposed to be different in different species, 

 or with respect to different hereditary characters, they are known also 

 as inheritance units. 



It is assumed in practically all theories of heredity that the " inher- 



