as many cells as there are nuclei in the tread. Hence, in the
fertilised egg which we eat daily, the yellow yelk is already a
multicellular body. Its tread is composed of several cells, and is now
commonly called the germinal disc. We shall return to this
discogastrula in Chapter 1.9.

(FIGURE 1.14. The human ovum, taken from the female ovary, magnified
500 times. The whole ovum is a simple round cell. The chief part of
the globular mass is formed by the nuclear yelk (deutoplasm), which is
evenly distributed in the active protoplasm, and consists of numbers
of fine yelk-granules. In the upper part of the yelk is the
transparent round germinal vesicle, which corresponds to the nucleus.
This encloses a darker granule, the germinal spot, which shows a
nucleolus. The globular yelk is surrounded by the thick transparent
germinal membrane (ovolemma, or zona pellucida). This is traversed by
numbers of lines as fine as hairs, which are directed radially towards
the centre of the ovum. These are called the pore-canals; it is
through these that the moving spermatozoa penetrate into the yelk at
impregnation.

FIGURE 1.15. A fertilised ovum from the oviduct of a hen. the yellow
yelk (c) consists of several concentric layers (d), and is enclosed in
a thin yelk-membrane (a). The nucleus or germinal vesicle is seen
above in the cicatrix or "tread" (b). From that point the white yelk
penetrates to the central yelk-cavity (d apostrophe). The two kinds of
yelk do not differ very much.

FIGURE 1.16. A creeping amoeba (highly magnified). The whole organism
is a simple naked cell, and moves about by means of the changing arms
which it thrusts out of and withdraws into its protoplasmic body.