When it emerges from the
shell, it is fully developed, and in a short time it begins an
independent existence, seeking and finding its own food. The fact that
eggs store so much nutritive material explains to some extent why they
are a valuable source of food for man and why they are used so
extensively. However, as in the case of milk, the elements that eggs
contain are not in just the right proportion for the sole nourishment of
a human being, so they must generally be used in combination with
other foods.
2. Most persons are familiar with the appearance of eggs, but in order
that satisfactory results may be obtained in their selection, care, and
cooking, it will be necessary to look into the details of their
composition. As is well known, an egg consists of a porous shell lined
with a fine, but tough, membrane that encloses the white and the yolk
and serves to protect them. The yolk is divided from the white by a
delicate membrane, which permits it to be separated from the white when
an egg is carefully broken. This membrane extends to each end of the
shell in the form of a small cord, and it is so fastened to the shell as
to hold the yolk evenly suspended. The porous nature of an egg shell is
required to give air to the developing chick, but it is this
characteristic that permits eggs to spoil as they grow old and are
exposed to air, for through these minute pores, or openings, the water
in the egg evaporates and air and bacteria enter. Of course, as the
water evaporates and is replaced by air, the egg becomes lighter.
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