attributing the cause to milk proteins, fats, carbohydrates, or salts.
Hopkins therefore suggested the existence of unknown factors in milk of
the type to which he had earlier given the name "accessory factors." This
work has recently been repeated by Osborne and Mendel who fail to find the
high potency in milk ascribed to it by Hopkins but the latter's work, at
that time, was accepted without question and became the impetus to
important discoveries.

Mendel and Osborne had meanwhile investigated more in detail their milk
fractions. They obtained results that confirmed McCollum's findings for
butter fat but in addition they showed that by removing all the fat and
protein from milk they obtained a residue which played an important part
in growth stimulation and that this factor was different from the salts
present in the mixture. This specially prepared milk residue they called
protein-free milk.

The next few years are a melting pot of investigations. They included some
sharp controversies over nomenclature and many apparently contradictory
conclusions based on what we now know to be insufficient data. The
principal outcome was the identification of the yeast and rice polishing
substance with the factor carried by protein-free milk. On the basis of
these results Funk put forward the idea that McCollum's butter-fat and
egg-yolk factor was merely vitamine which clung to the fats as an
adulterant. It was soon shown, however, that butter fat could be obtained
that was absolutely free of nitrogen and still be stimulatory to growth.
It was therefore clear that whatever the factor present it could not be
the Funk vitamine. From out of the smoke of this controversy came an
ultimate explanation that was very simple. There were two factors instead
of one. McCollum did not discover the presence of the Funk vitamine in his
mixtures at first because it was carried by the lactose and he did not