||That this newly
recognized structure must be important in the economy of the cell was recognized
by Brown himself, and by the celebrated German Meyen, who dealt with it
in his work on vegetable physiology, published not long afterwards; but
it remained for another German, the professor of botany in the University
of Jena, Dr. M. J. Schleiden, to bring the nucleus to popular attention,
and to assert its all-importance in the economy of the cell.
Schleiden freely acknowledged his indebtedness
to Brown for first knowledge of the nucleus, but he soon carried his studies
of that structure far beyond those of its discoverer. He came to believe
that the nucleus is really the most important portion of the cell, in that
it is the original structure from which the remainder of the cell is developed.
Hence he named it the cytoblast. He outlined his views in an epochal paper
published in Muller's Archives in 1838, under title of "Beitrage zur Phytogenesis."
This paper is in itself of value, yet the most important outgrowth of Schleiden's
observations of the nucleus did not spring from his own labors, but from
those of a friend to whom he mentioned his discoveries the year previous
to their publication. This friend was Dr. Theodor Schwann, professor of
physiology in the University of Louvain.
At the moment when these observations were
communicated to him Schwann was puzzling over certain details of animal
histology which he could not clearly explain. His great teacher, Johannes
Muller, had called attention to the strange resemblance to vegetable cells
shown by certain cells of the chorda dorsalis (the embryonic cord from
which the spinal column is developed), and Schwann himself had discovered
a corresponding similarity in the branchial cartilage of a tadpole. Then,
too, the researches of Friedrich Henle had shown that the particles that
make up the epidermis of animals are very cell-like in appearance. Indeed,
the cell-like character of certain animal tissues had come to be matter
of common note among students of minute anatomy. Schwann felt that this
similarity could not be mere coincidence, but he had gained no clew to
further insight until Schleiden called his attention to the nucleus. Then
at once he reasoned that if there really is the correspondence between
vegetable and animal tissues that he suspected, and if the nucleus is so
important in the vegetable cell as Schleiden believed, the nucleus should
also be found in the ultimate particles of animal tissues.
Schwann's researches soon showed the entire
correctness of this assumption. A closer study of animal tissues under
the microscope showed, particularly in the case of embryonic tissues, that
"opaque spots" such as Schleiden described are really to be found there
in abundance - forming, indeed, a most characteristic phase of the structure.
The location of these nuclei at comparatively regular intervals suggested
that they are found in definite compartments of the tissue, as Schleiden
had shown to be the case with vegetables; indeed, the walls that separated
such cell-like compartments one from another were in some cases visible.
Particularly was this found to be the case with embryonic tissues, and
the study of these soon convinced Schwann that his original surmise had
been correct, and that all animal tissues are in their incipiency composed
of particles not unlike the ultimate particles of vegetables in short,
of what the botanists termed cells. Adopting this name, Schwann propounded
what soon became famous as his cell theory, under title of Mikroskopische
Untersuchungen uber die Ubereinstimmung in der Structur und dent Wachsthum
der Thiere und Pflanzen. So expeditious had been his work that this book
was published early in 1839, only a few months after the appearance of
As the title suggests, the main idea that
actuated Schwann was to unify vegetable and animal tissues. Accepting cell-structure
as the basis of all vegetable tissues, he sought to show that the same
is true of animal tissues, all the seeming diversities of fibre being but
the alteration and development of what were originally simple cells. And
by cell Schwann meant, as did Schleiden also, what the word ordinarily
implies - a cavity walled in on all sides. He conceived that the ultimate
constituents of all tissues were really such minute cavities, the most
important part of which was the cell wall, with its associated nucleus.
He knew, indeed, that the cell might be filled with fluid contents, but
he regarded 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