represented, on
the one hand, by the Stahlian doctrine, supported by Liebig, according to
which the atoms of the sugar are shaken into new combinations either
directly by the _Toruloe_, or indirectly, by some substance formed by
them; and, on the other hand, by the Thenardian doctrine, supported by
Pasteur, according to which the yeast plant assimilates part of the
sugar, and, in so doing, disturbs the rest, and determines its resolution
into the products of fermentation. Perhaps the two views are not so much
opposed as they seem at first sight to be.

But the interest which attaches to the influence of the yeast plants upon
the medium in which they live and grow does not arise solely from its
bearing upon the theory of fermentation. So long ago as 1838, Turpin
compared the _Toruloe_ to the ultimate elements of the tissues of animals
and plants--"Les organes elementaires de leurs tissus, comparables aux
petits vegetaux des levures ordinaires, sont aussi les decompositeurs des
substances qui les environnent."

Almost at the same time, and, probably, equally guided by his study of
yeast, Schwann was engaged in those remarkable investigations into the
form and development of the ultimate structural elements of the tissues
of animals, which led him to recognise their fundamental identity with
the ultimate structural elements of vegetable organisms.

The yeast plant is a mere sac, or "cell," containing a semi-fluid matter,
and Schwann's microscopic analysis resolved all living organisms, in the
long run, into an aggregation of such sacs or cells, variously modified;
and tended to show, that all, whatever their ultimate complication, begin
their existence in the condition of such simple cells.

In his famous "Mikroskopische Untersuchungen" Schwann speaks of _Torula_
as a "cell"; and, in a remarkable note to the passage in which he refers
to the yeast plant, Schwann says:--

"I have been unable to avoid mentioning fermentation, because it is the
most fully and exactly known operation of cells, and represents, in the
simplest fashion, the process which is repeated by every cell of the
living body."

In other words, Schwann conceives that every cell of the living body
exerts an influence on the matter which surrounds and permeates it,
analogous to that which a _Torula_ exerts on the saccharine solution by
which it is bathed. A wonderfully suggestive thought, opening up views of
the nature of the chemical processes of the