"Verum talis mini de horum origine et formatione conceptus
formabam; globulis nempe ex quibus farina Tritici, Hordei,
Avenae, Fagotritici, se constat aquae calore dissolvi et aquae
commisceri; hac, vero aqua, quam cerevisiam vocare licet,
refrigescente, multos ex minimis particulis in cerevisia
coadunari, et hoc pacto efficere particulam sive globulum,
quae sexta pars est globuli faecis, et iterum sex ex hisce
globulis conjungi."[1]

[Footnote 1: Leeuwenhoek, "Arcana Naturae Detecta." Ed. Nov., 1721.]

Thus Leeuwenhoek discovered that yeast consists of globules floating
in a fluid; but he thought that they were merely the starchy particles
of the grain from which the wort was made, re-arranged. He discovered
the fact that yeast had a definite structure, but not the meaning of
the fact. A century and a half elapsed, and the investigation of
yeast was recommenced almost simultaneously by Cagniard de la Tour in
France, and by Schwann and Kützing in Germany. The French observer
was the first to publish his results; and the subject received at his
hands and at those of his colleague, the botanist Turpin, full and
satisfactory investigation.

The main conclusions at which they arrived are these. The globular,
or oval, corpuscles which float so thickly in the yeast as to make it
muddy, though the largest are not more than one two-thousandth of
an inch in diameter, and the smallest may measure less than one
seven-thousandth of an inch, are living organisms. They multiply with
great rapidity, by giving off minute buds, which soon attain the size
of their parent, and then either become detached or remain united,