[Fig. 3]
Fig. 3--Diagram showing the relation 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.
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