The fertilised bird-ovum (Figure 1.15) is notably different. It is
true that in its earliest stage (Figure 1.13 E) this ovum also is very
like that of the mammal (Figure 1.13 F). But afterwards, while still
within the oviduct, it takes up a quantity of nourishment and works
this into the familiar large yellow yelk. When we examine a very young
ovum in the hen's oviduct, we find it to be a simple, small, naked,
amoeboid cell, just like the young ova of other animals (Figure 1.13).
But it then grows to the size we are familiar with in the round yelk
of the egg. The nucleus of the ovum, or the germinal vesicle, is thus
pressed right to the surface of the globular ovum, and is embedded
there in a small quantity of transparent matter, the so-called white
yelk. This forms a round white spot, which is known as the "tread"
(cicatricula) (Figure 1.15 b). From the tread a thin column of the
white yelk penetrates through the yellow yelk to the centre of the
globular cell, where it swells into a small, central globule (wrongly
called the yelk-cavity, or latebra, Figure 1.15 d apostrophe). The
yellow yelk-matter which surrounds this white yelk has the appearance
in the egg (when boiled hard) of concentric layers (c). The yellow
yelk is also enclosed in a delicate structureless membrane (the
membrana vitellina, a).

As the large yellow ovum of the bird attains a diameter of several
inches in the bigger birds, and encloses round yelk-particles, there
was formerly a reluctance to consider it as a simple cell. This was a
mistake. Every animal that has only one cell-nucleus, every amoeba,
every gregarina, every infusorium, is unicellular, and remains
unicellular whatever variety of matter it feeds on. So the ovum
remains a simple cell, however much yellow yelk it afterwards
accumulates within its protoplasm. It is, of course, different, with
the bird's egg when it has been fertilised. The ovum then consists of