It had, indeed, long been recognized that the tissues of plants were
to a large extent composed of minute vesicular bodies, technically
called cells (Hooke, Malpighi, Grew). In 1831 the discovery was made
by the great botanist, Robert Brown, that in many families of plants a
circular spot, which he named areola or nucleus, was present in each
cell; and in 1838 M. J. Schleiden published the fact that a similar
spot or nucleus was a universal elementary organ in vegetables. In
the tissues of animals also structures had begun to be recognized
comparable with the cells and nuclei of the vegetable tissues, and in
1839 Theodore Schwann announced the important generalization that
there is one universal principle of development for the elementary
part of organisms, however different they may be in appearance, and
that this principle is the formation of cells. The enunciation of the
fundamental principle that the elementary tissues consisted of cells
constituted a step in the progress of biological science which will
forever stamp the century now drawing to a close with a character and
renown equalling those which it has derived from the most brilliant
discoveries in the physical sciences. It provided biologists with the
visible anatomical units through which the external forces operating
on, and the energy generated in, living matter come into play. It
dispelled forever the old mystical idea of the influence exercised by
vapors or spirits in living organisms. It supplied the physiologist and
pathologist with the specific structures through the agency of which
the functions of organisms are discharged in health and disease. It
exerted an enormous influence on the progress of practical medicine. A
review of the progress of knowledge of the cell may appropriately enter
into an address on this occasion.