free fluid without any external pressure. In such cases the nucleus
also is not infrequently round, and located in the centre of the
cell-body (Figure 1.2 k). In other cases, the cells have no definite
shape; they are constantly changing their form owing to their
automatic movements. This is the case with the amoebae (Figures 1.15
and 1.16) and the amoeboid travelling cells (Figure 1.11), and also
with very young ova (Figure 1.13). However, as a rule, the cell
assumes a definite form in the course of its career. In the tissues of
the multicellular organism, in which a number of similar cells are
bound together in virtue of certain laws of heredity, the shape is
determined partly by the form of their connection and partly by their
special functions. Thus, for instance, we find in the mucous lining of
our tongue very thin and delicate flat cells of roundish shape (Figure
1.3). In the outer skin we find similar, but harder, covering cells,
joined together by saw-like edges (Figure 1.4). In the liver and other
glands there are thicker and softer cells, linked together in rows
(Figure 1.5).

The last-named tissues (Figures 1.3 to 1.5) belong to the simplest and
most primitive type, the group of the "covering-tissues," or
epithelia. In these "primary tissues" (to which the germinal layers
belong) simple cells of the same kind are arranged in layers. The
arrangement and shape are more complicated in the "secondary tissues,"
which are gradually developed out of the primary, as in the tissues of
the muscles, nerves, bones, etc. In the bones, for instance, which
belong to the group of supporting or connecting organs, the cells
(Figure 1.6) are star-shaped, and are joined together by numbers of
net-like interlacing processes; so, also, in the tissues of the teeth
(Figure 1.7), and in other forms of supporting-tissue, in which a soft
or hard substance (intercellular matter, or base) is inserted between