ged in a
definite manner in which, in every single part, in the leaves as in the
root, in the trunk as in the blossom, cells are discovered to be the
ultimate elements, so it is with the forms of animal life. Every animal
presents itself as a sum of vital unities, every one of which manifests
all the characteristics of life. The characteristics and unity of life
cannot be limited to any one particular spot in an organism (for
instance, to the brain of a man) but are to be found only in the
definite, constantly recurring structure, which every individual element
displays. A so-called individual always represents an arrangement of a
social kind, in which a number of individual existences are mutually
dependent, but in such a way that every element has its own special
action, and even though it derive its stimulus to activity from other
parts, yet alone affects the actual performance of its duties.

Between cells there is a greater or less amount of a homogeneous
substance--the _intercellular substance_. According to Schwann, the
intercellular substance was cyto-blastema destined for the development
of new cells; I believe this is not so, I believe that the intercellular
substance is dependent in a certain definite manner upon the cells, and
that certain parts of it belong to one cell and parts to another.

At various times, fibres, globules, and elementary granules, have been
regarded as histological starting-points. Now, however, we have
established the general principle that no development of any kind begins
_de novo_ and that as spontaneous generation is impossible in the case
of entire organisms, so also it is impossible in the case of individual
parts. No cell can build itself up out of non-cellular material. Where a
cell arises, there a cell must have previously existed (omnis cellula e
cellula), just as an animal can spring only from an animal, and a plant
only from a plant. No developed tissues can be traced back to anything
but a cell.

If we wish to classify tissues, a very simple division offers itself. We
have (a) tissues which consist exclusively of cells, where cell lies
close to cell. (b) Tissues in which the cells are separated by a certain
amount of intercellular substance. (c) Tissues of a high or peculiar
type, such as the nervous and muscular systems and vessels. An example
of the first class is seen in the _epithelial_ tissues. In these, cell
lies close to cell, with nothing between.

The second c