for believing that it may be
formed out of other parts of the cell. The nucleus is always a direct
descendant from the nucleus of pre-existing cells, so that there is an
absolute continuity of descent between the nucleii of the cells of an
individual and those of its antecedents back for numberless generations.
It is not certain that there is any such continuity of descent in the
case of the centrosomes; for, while in the process of fertilization the
centrosome is handed down from parent to child, there are some reasons
for believing that it may disappear in subsequent cells, and later be
redeveloped out of other parts. The only part of the cell in which
complete continuity from parent to child is demonstrated, is the nucleus
and particularly the chromosomes. All of these facts simply emphasize
the importance of the chromosomes, and tell us that these bodies must be
regarded as containing the most important features of the cell which
constitute its individuality.

==What is Protoplasm?==--Enough has now been given of disclosures of the
modern microscope to show that our old friend Protoplasm has assumed an
entirely new guise, if indeed it has not disappeared altogether. These
simplest life processes are so marvelous and involve the action of such
an intricate mass of machinery that we can no longer retain our earlier
notion of protoplasm as the physical basis of life. There can be no life
without the properties of assimilation, growth, and reproduction; and,
so far as we know, these properties are found only in that combination
of bodies which we call the cell, with its mixture of harmoniously
acting parts. _Life, at least the life of a cell, is then not the
property of a chemical compound protoplasm, but is the result of the
activities of a machine._ Indeed, we are now at a loss to know how we
can retain the term protoplasm. As originally used it meant the contents
of the cell, and the significance in the term was in the conception of
protoplasm as a somewhat homogeneous chemical compound uniform in all
types of life. But we now see that this cell contains not a single
substance, but a large number, including solids, jelly masses, and
liquids, each of which has its own chemical composition. The number of
chemical compounds existing in the material formerly called protoplasm
no one knows, but we do know that they are many, and that the different
substances are combined to form a physical structure. Which of these
variou