body must, therefore, be brought about by
other means. Such a means is found in the formation of new cells, or _cell
reproduction_. The new cells are always formed _by_ and _from_ the old
cells, the essential process being known as _cell-division_.

[Fig. 5]

Fig. 5--Steps in cell-division (after Wilson). Note that the process begins
with the division of the attraction sphere, then involves the nucleus, and
finally separates the main body.

*Cell-Division.*--By dividing, a single cell will, on attaining its growth,
separate into two or more new cells. The process is quite complex and is
imperfectly understood. It is known, however, that the act of separation
is preceded by a series of changes in which the attraction sphere and the
nucleus actively participate, and that, as a result of these changes, the
contents of the old cell are rearranged to form the new cells. Some of the
different stages in the process, as they have been studied under the
microscope, are indicated in Fig. 5.

Gradually, through the formation of new cells and by the growth of these
cells after they have been formed, the body attains its full size. When
growth is complete, cell reproduction is supposed to cease except where
the tissues are injured, as in the breaking of a bone, or where cells,
like those at the surface of the skin, are subject to wear. Then new
material continues to be added to the protoplasm throughout life, but in
amount only sufficient to replace that lost from the protoplasm as waste.

[Fig. 6]

Fig. 6--A tumbler partly filled with marbles covered with water, suggesting
the relations of the cells to the lymph.

*Cell Surroundings.*--All cells are said to be _aquatic_. This means simply
that they require water for carrying on their various activities. The
cells, in order to live, must take in and give out materials, and water is
necessary to both processes. It is also an essential part of the
protoplasm. Deprived of water, cells become inactive and usually die.
Aquatic surroundings are provided for the cells of the body through a
liquid known as the _lymph_, which is distributed throughout the
intercellular material (Fig. 6). This consists of water containing oxygen
and food substances in solution. Besides supplying these to the cells, the
lymph also receives their wastes. Through the lymph the necessary
condition