secondary cells lying free in the cavity of a
cell."

But gradually the researches of such accurate observers as Unger,
Nageli, Kolliker, Reichart, and Remak tended to confirm the opinion of
Von Mohl that cells spring only from cells, and finally Rudolf Virchow
brought the matter to demonstration about 1860. His Omnis cellula e
cellula became from that time one of the accepted data of physiology.
This was supplemented a little later by Fleming's Omnis nucleus e
nucleo, when still more refined methods of observation had shown that
the part of the cell which always first undergoes change preparatory to
new cell-formation is the all-essential nucleus. Thus the nucleus was
restored to the important position which Schwann and Schleiden had given
it, but with greatly altered significance. Instead of being a structure
generated de novo from non-cellular substance, and disappearing as soon
as its function of cell-formation was accomplished, the nucleus was now
known as the central and permanent feature of every cell, indestructible
while the cell lives, itself the division-product of a pre-existing
nucleus, and the parent, by division of its substance, of other
generations of nuclei. The word cell received a final definition as "a
small mass of protoplasm supplied with a nucleus."

In this widened and culminating general view of the cell theory it
became clear that every animate organism, animal or vegetable, is but a
cluster of nucleated cells, all of which, in each individual case, are
the direct descendants of a single primordial cell of the ovum. In the
developed individuals of higher organisms the successive generations of
cells become marvellously diversified in form and in specific functions;
there is a wonderful division of labor, special functions being chiefly
relegated to definite groups of cells; but from first to last there is
no function developed that is not present, in a primitive way, in
every cell, however isolated; nor does the developed cell, however
specialized, ever forget altogether any one of its primordial functions
or capacities. All physiology, then, properly interpreted, becomes
merely a study of cellular activities; and the development of the cell
theory takes its place as the great central generalization in physiology
of the nineteenth century. Something of the later developments of this
theory we shall see in another connection.

ANIMAL CHEMISTRY

Just at the time when the microscope was op