Liebig, according to which the atoms of the sugar are shaken into new
combinations, either directly by the _Torulae_, or indirectly, by some
substance formed by them; and, on the other hand, by the Thénardian
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 _Torulae_ to the ultimate elements of the tissues
of animals and plants--"Les organes élémentaires de leurs tissus,
comparables aux petits végétaux des levures ordinaires, sont aussi les
décompositeurs 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 recognize 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.