classification. Palaeontology is, unfortunately, altogether neglected
by many of these new workers, although this interesting science
furnishes most important facts for phylogeny, and thus often proves of
very great service in ontogeny.

A very important advance was made in our science in 1839, when the
cellular theory was established, and a new field of inquiry bearing on
embryology was suddenly opened. When the famous botanist, M.
Schleiden, of Jena, showed in 1838, with the aid of the microscope,
that every plant was made up of innumerable elementary parts, which we
call cells, a pupil of Johannes Muller at Berlin, Theodor Schwann,
applied the discovery at once to the animal organism. He showed that
in the animal body as well, when we examine its tissues in the
microscope, we find these cells everywhere to be the elementary units.
All the different tissues of the organism, especially the very
dissimilar tissues of the nerves, muscles, bones, external skin,
mucous lining, etc., are originally formed out of cells; and this is
also true of all the tissues of the plant. These cells are separate
living beings; they are the citizens of the State which the entire
multicellular organism seems to be. This important discovery was bound
to be of service to embryology, as it raised a number of new
questions. What is the relation of the cells to the germinal layers?
Are the germinal layers composed of cells, and what is their relation
to the cells of the tissues that form later? How does the ovum stand
in the cellular theory? Is the ovum itself a cell, or is it composed
of cells? These important questions were now imposed on the
embryologist by the cellular theory.

The most notable effort to answer these questions--which were attacked
on all sides by different students--is contained in the famous work,