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
conditions for cell life are provided in the body.

*The General Work of Cells.*—In handling the materials derived from the
lymph, the cells carry on three well-defined processes, known as
absorption, assimilation, and excretion.