ed these as relatively subordinate in importance to the wall
itself. This, however, did not apply to the nucleus, which was supposed
to lie against the cell wall and in the beginning to generate it.
Subsequently the wall might grow so rapidly as to dissociate itself
from its contents, thus becoming a hollow bubble or true cell; but the
nucleus, as long as it lasted, was supposed to continue in contact
with the cell wall. Schleiden had even supposed the nucleus to be a
constituent part of the wall, sometimes lying enclosed between two
layers of its substance, and Schwann quoted this view with seeming
approval. Schwann believed, however, that in the mature cell the nucleus
ceased to be functional and disappeared.

The main thesis as to the similarity of development of vegetable and
animal tissues and the cellular nature of the ultimate constitution
of both was supported by a mass of carefully gathered evidence which a
multitude of microscopists at once confirmed, so Schwann's work became
a classic almost from the moment of its publication. Of course various
other workers at once disputed Schwann's claim to priority of discovery,
in particular the English microscopist Valentin, who asserted, not
without some show of justice, that he was working closely along the same
lines. Put so, for that matter, were numerous others, as Henle, Turpin,
Du-mortier, Purkinje, and Muller, all of whom Schwann himself had
quoted. Moreover, there were various physiologists who earlier than
any of these had foreshadowed the cell theory--notably Kaspar Friedrich
Wolff, towards the close of the previous century, and Treviranus about
1807, But, as we have seen in so many other departments of science, it
is one thing to foreshadow a discovery, it is quite another to give
it full expression and make it germinal of other discoveries. And when
Schwann put forward the explicit claim that "there is one universal
principle of development for the elementary parts, of organisms, however
different, and this principle is the formation of cells," he enunciated
a doctrine which was for all practical purposes absolutely new and
opened up a novel field for the microscopist to enter. A most important
era in physiology dates from the publication of his book in 1839.

THE CELL THEORY ELABORATED

That Schwann should have gone to embryonic tissues for the establishment
of his ideas was no doubt due very largely to the influence of the great
Russian Karl Ernst von