come to have a biological meaning and application.

* * * * *

The cell, then, is in all respects the very unit of the organic world. Not
only is it the ultimate structural element of all the more familiar
animals and plants that we know, as the foregoing analysis demonstrates,
but, in the second place, the microscope reveals simple little organisms,
like _Amoeba_, the yeast plant and bacteria, which consist throughout
their lives of just one cell and nothing more. Still more wonderful is the
fact that the larger complex organisms actually begin existence as single
cells. In three ways, therefore,--the analytic, the comparative, and the
developmental,--the cell proves to be the "organic individual of the first
order." As the ultimate biological unit, its essential nature must possess
a profound interest, for in its substance resides the secret of life.

This wonderful physical basis of life is called _protoplasm_. It contains
three kinds of chemical compounds known as the proteins, carbohydrates,
and hydrocarbons. Proteins are invariably present in living cells, and are
made up of carbon, hydrogen, nitrogen, sulphur, and usually a little
phosphorus. The elements are also combined in a very complex chemical way.
For example, the substance called haemoglobin is the protein which exists
in the red blood cells and which causes those cells to appear light red or
yellow when seen singly. Its chemical formula states the precise number of
atoms which enter into the constitution of a single molecule as:
C_{600}H_{960}N_{154}FeO_{179}. This is truly a marvelously complex
substance when compared with the materials of the inorganic world, like
water, for example, which has the formula H_{2}O. And just as the peculiar
properties of H_{2}O are given to it by the properties of the hydrogen and
the oxygen which combine to form it, just so, the scientist believes, the
marvelous properties of protein are due to the assemblage of the
properties of the carbon and hydrogen and other elements which enter into
its composition.

It would be interesting to see how each one of these elements contributes
some particular characteristic to the whole compound. The carbon atom, for
example, is prone to combine with other atoms in definite varied ways, and
the high degree of complexity which the protein molecule possesses may
depend in greater part upon the combining power of its carbon elements.
The nitrogen atom mak