the intercellular matter. The internal plasma-products are either the
fluid cell-sap or hard structures. As a rule, in mature and
differentiated cells these various parts are so arranged that the
protoplasm (like the caryoplasm in the round nucleus) forms a sort of
skeleton or framework. The spaces of this network are filled partly
with the fluid cell-sap and partly by hard structural products.

(FIGURE 1.6. Nine star-shaped bone-cells, with interlaced branches.

FIGURE 1.7. Eleven star-shaped cells from the enamel of a tooth,
joined together by their branchlets.)

The simple round ovum, which we take as the starting-point of our
study (Figures 1.1 and 1.2), has in many cases the vague, indifferent
features of the typical primitive cell. As a contrast to it, and as an
instance of a very highly differentiated plastid, we may consider for
a moment a large nerve-cell, or ganglionic cell, from the brain. The
ovum stands potentially for the entire organism--in other words, it
has the faculty of building up out of itself the whole multicellular
body. It is the common parent of all the countless generations of
cells which form the different tissues of the body; it unites all
their powers in itself, though only potentially or in germ. In
complete contrast to this, the neural cell in the brain (Figure 1.9)
develops along one rigid line. It cannot, like the ovum, beget endless
generations of cells, of which some will become skin-cells, others
muscle-cells, and others again bone-cells. But, on the other hand, the
nerve-cell has become fitted to discharge the highest functions of
life; it has the powers of sensation, will, and thought. It is a real
soul-cell, or an elementary organ of the psychic activity. It has,
therefore, a most elaborate and delicate structure. Numbers of