but the essential point in our view of the cell is to look upon it as
a self-contained or independent living unit. It is, in the words of
Brucke, "an elementary organism." We may define it most precisely as
the ultimate organic unit, and, as the cells are the sole active
principles in every vital function, we may call them the "plastids,"
or "formative elements." This unity is found in both the anatomic
structure and the physiological function. In the case of the protists,
the entire organism usually consists of a single independent cell
throughout life. But in the tissue-forming animals and plants, which
are the great majority, the organism begins its career as a simple
cell, and then grows into a cell-community, or, more correctly, an
organised cell-state. Our own body is not really the simple unity that
it is generally supposed to be. On the contrary, it is a very
elaborate social system of countless microscopic organisms, a colony
or commonwealth, made up of innumerable independent units, or very
different tissue-cells.

In reality, the term "cell," which existed long before the cell theory
was formulated, is not happily chosen. Schleiden, who first brought it
into scientific use in the sense of the cell theory, gave this name to
the elementary organisms because, when you find them in the dissected
plant, they generally have the appearance of chambers, like the cells
in a bee-hive, with firm walls and a fluid or pulpy content. But some
cells, especially young ones, are entirely without the enveloping
membrane, or stiff wall. Hence we now generally describe the cell as a
living, viscous particle of protoplasm, enclosing a firmer nucleus in
its albuminoid body. There may be an enclosing membrane, as there
actually is in the case of most of the plants; but it may be wholly
lacking, as is the case with most of the animals. There is no membrane
at all in the first stage. The young cells are usually round, but they