individuality. This cell produces a cluster of cells by segmentation,
and from these develops the multicellular organism, or individual of
higher rank.

When we examine a little closer the original features of the ovum, we
notice the extremely significant fact that in its first stage the ovum
is just the same simple and indefinite structure in the case of man
and all the animals (Figure 1.13). We are unable to detect any
material difference between them, either in outer shape or internal
constitution. Later, though the ova remain unicellular, they differ in
size and shape, enclose various kinds of yelk-particles, have
different envelopes, and so on. But when we examine them at their
birth, in the ovary of the female animal, we find them to be always of
the same form in the first stages of their life. In the beginning each
ovum is a very simple, roundish, naked, mobile cell, without a
membrane; it consists merely of a particle of cytoplasm enclosing a
nucleus (Figure 1.13). Special names have been given to these parts of
the ovum; the cell-body is called the yelk (vitellus), and the
cell-nucleus the germinal vesicle. As a rule, the nucleus of the ovum
is soft, and looks like a small pimple or vesicle. Inside it, as in
many other cells, there is a nuclear skeleton or frame and a third,
hard nuclear body (the nucleolus). In the ovum this is called the
germinal spot. Finally, we find in many ova (but not in all) a still
further point within the germinal spot, a "nucleolin," which goes by
the name of the germinal point. The latter parts (germinal spot and
germinal point) have, apparently, a minor importance, in comparison
with the other two (the yelk and germinal vesicle). In the yelk we
must distinguish the active formative yelk (or protoplasm = first
plasm) from the passive nutritive yelk (or deutoplasm = second plasm).