or of groups of cells. The single cells live independently of one another,
but the cells that form groups are attached to, and are more or less
dependent upon, one another. In the first condition are found the very
lowest forms of life. In the second, life reaches its greatest
development. The body of man, which represents the highest type of life,
is recognized as a group of cells. In this group each cell is usually
separate and distinct from the others, but is attached to them, and is
held in place by the intercellular material.

*Protoplasm, the Cell Substance.*—The cell is properly regarded as an
_organized_ bit of a peculiar material, called _protoplasm_. This is a
semi-liquid and somewhat granular substance which resembles in appearance
the white of a raw egg. Its true nature and composition are unknown,
because any attempt to analyze it kills it, and dead protoplasm is
essentially different from living protoplasm. It is known, however, to be
a highly complex substance and to undergo chemical change readily. It
appears to be the only kind of matter with which life is ever associated,
and for this reason protoplasm is called the _physical basis of life_. Its
organization into separate bits, or cells, is necessary to the life
activities that take place within it.

*Structure of the Cell.*—Though all portions of the cell are formed from
the protoplasm, this essential substance differs both in structure and in
function at different places in the cell. For this reason the cell is
looked upon as a complex body having several distinct parts. At or near
the center is a clear, rounded body, called the _nucleus_. This plays some
part in the nourishment of the cell and also in the formation of new
cells. If it be absent, as is sometimes the case, the cell is short-lived
and unable to reproduce itself. The variety of protoplasm contained in the
nucleus is called the _nucleoplasm_.