of the cells and the intercellular
material. _C._ Cells. _I._ Intercellular material.

*Complex Nature of the Tissues.*--To the unaided eye the tissues have the
appearance of simple structures. The microscope, however, shows just the
reverse to be true. When any one of the tissues is suitably prepared and
carefully examined with this instrument, at least two classes of materials
can be made out. One of these consists of minute particles, called
_cells_; the other is a substance lying between the cells, known as the
_intercellular material_ (Fig. 3). The cells and the intercellular
material, though varying in their relative proportions, are present in all
the tissues.

*The Body a Cell Group.*--The biologist has found that the bodies of all
living things, plants as well as animals, consist either of single cells
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, calle