lt 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 Schleiden's
paper.

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 regard