r the sake of
the parts. But when Turpin and Schwann resolved the living body into
an aggregation of quasi-independent cells, each, like a _Torula_,
leading its own life and having its own laws of growth and
development, the aggregation being dominated and kept working towards
a definite end only by a certain harmony among these units, or by the
superaddition of a controlling apparatus, such as a nervous system,
this conception ceased to be tenable. The cell lives for its own sake,
as well as for the sake of the whole organism; and the cells, which
float in the blood, live at its expense, and profoundly modify it, are
almost as much independent organisms as the _Torulae_ which float in
beer-wort.

Schwann burdened his enunciation of the "cell theory" with two false
suppositions; the one, that the structures he called "nucleus" and
"cell-wall" are essential to a cell; the other, that cells are usually
formed independently of other cells; but, in 1839, it was a vast and
clear gain to arrive at the conception, that the vital functions of
all the higher animals and plants are the resultant of the forces
inherent in the innumerable minute cells of which they are composed,
and that each of them is, itself, an equivalent of one of the lowest
and simplest of independent living beings--the _Torula._

From purely morphological investigations, Turpin and Schwann, as we
have seen, arrived at the notion of the fundamental unity of structure
of living beings. And, before long, the researches of chemists
gradually led up to the conception of the fundamental unity of their
composition.

So far back as 1803, Thenard pointed out, in most distinct terms, the
important fact that yeast contains a nitrogenous "animal" substance;
and that such a substance is contained in all ferments. Before him,
Fabroni and Fourcroy speak of the "vegeto-animal" matter of yeast.
In 1844 Mulder endeavoured to demonstrate that a peculiar substance,
which he called "protein," was essentially characteristic of living
matter. In 1846, Payen writes:--

"Enfin, une loi sans exception me semble apparaitre dans les
faits nombreux que j'ai observes et conduire a envisager sous
un nouveau jour la vie vegetale; si je ne m'abuse, tout ce
que dans les tissus vegetaux la vue directe ou amplifiee nous
permet de discerner sous la forme de cellules et de vaisseaux,
ne represente autre chose que les enveloppes protectrices,
les reservoirs et le