inative cells alone of all its parts continue its
life under certain conditions.

=The Cells: Protoplasm. The Nucleus.=--Since the time of
_Schwann_ (1830) it is agreed that the cell is the most simple
morphological element which is capable of living. Among the lower
organisms this element constitutes the entire individual. There is no
doubt that the cell is already a thing of high organization. It is
formed of infinitely small elements of very different value and
chemical constitution, which form what is called _protoplasm_ or the
cell-substance. But these infinitely small elements are so far
absolutely unknown. It is in them that must be sought the change from
inanimate matter, that is the chemical molecule, to living matter, a
change which was formerly believed to lie in the protoplasm itself,
before its complicated structure was known. We need not concern
ourselves here with this question which remains an open one.

Life being established, the cell remains its only known constant
element. The cell is composed of protoplasm which contains a rounded
nucleus formed of _nucleo-plasma_. The nucleus is the most important
part of the cell, and governs its life.

=Cell-division.=--The lowest unicellular organisms, as each cell of a
multicellular organism, reproduce themselves by division or
_fission_. Each cell originates from another cell in the following
manner: the cell divides in the center as well as its nucleus, and in
this way forms two cells which grow by absorbing by _endosmosis_
(filtration) the nutritive juices which surround them. Death or
destruction of the cell is therefore death of the entire organism when
this is unicellular. But it has been previously reproduced.

We find here already the special and fundamental act of conjugation,
that is the fusion of two cells into one, which serves to strengthen
reproduction. This act, common to all living things including man,
shows us that continuation of life is only possible when from time to
time different elements, that is elements which have been exposed to
different influences, combine together. If this conjugation is
prevented and life is allowed to continue indefinitely by means of
fission or by budding (_vide infra_), there results a progressive
weakening and degeneration which leads to the disappearance of the
whole group thus reproduced.

It is necessary to explain here the results of recent scientific work
on the intimate phenomena of cell-division,