Page:Popular Science Monthly Volume 85.djvu/112

108 to generation, and whose characteristics, as compared with the general protoplasm, are great stability, independence and continuity. This is the idioplasm of Nägeli, 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 independent of the general protoplasm is a mere fiction which finds no justification in reality.

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 processes. 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 centrosomes, 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 chemistry. The smallest particle which can be seen with the most powerful microscope in ordinary light is about 250μμ (millionths of a millimeter) in diameter. The largest molecules are probably about 10μμ in diameter. Between these invisible molecules and the just visible particles 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 "