In a laying hen may be found, upon opening the body, what is called the
_ovarium_--a cluster of rudimental eggs, of different sizes, from very
minute points up to shapes of easily-distinguished forms. These
rudimental eggs have as yet no shell or white, these being exhibited in
a different stage of development; but consist wholly of _yolk_, on the
surface of which the germ of the future chicken lies. The yolk and the
germ are enveloped by a very thin membrane.
When the rudimental egg, still attached to the ovarium, becomes longer
and larger, and arrives at a certain size, either its own weight, or
some other efficient cause, detaches it from the cluster, and makes it
fall into a sort of funnel, leading to a pipe, which is termed the
_oviduct_.
Here the yolk of the rudimental egg, hitherto imperfectly formed, puts
on its mature appearance of a thick yellow fluid; while the rudimental
chick or embryo, lying on the surface opposite to that by which it had
been attached to the ovarium, is white, and somewhat like paste.
The white, or _albumen_, of the egg now becomes diffused around the
yolk, being secreted from the blood vessels of the egg-pipe, or oviduct,
in the form of a thin, glassy fluid; and it is prevented from mixing
with the yolk and the embryo chicken by the thin membrane which
surrounded them before they were detached from the egg-cluster, while
it is strengthened by a second and stronger membrane, formed around the
first, immediately after falling into the oviduct. This second membrane,
enveloping the yolk of the germ of the chicken, is thickest at the two
ends, having what may be termed bulgings, termed _chalazes_ by
anatomists; these bulgings of the second membrane pass quite through the
white at the ends, and being thus, as it were, embedded in the white,
they keep the inclosed yolk and germ somewhat in a fixed position,
preventing them from rolling about within the egg when it is moved.
The white of the egg being thus formed, a third membrane, or, rather, a
double membrane, much stronger than either of the first two, is formed
around it, becoming attached to the chalazes of the second membrane, and
tending still more to keep all the parts in their relative positions.
During the progress of these several formations, the egg gradually
advances about half way along the oviduct. It is still, however,
destitute of the shell, which begins to be formed by a process similar
to the formation of the shell of a snail, as soon as the outer layer of
the third membrane has been completed. When the shell is fully formed,
the egg continues to advance along the oviduct, till the hen goes to her
nest and lays it.
From ill health, or accidents, eggs are sometimes excluded from the
oviducts before the shell has begun to be formed, and in this state they
are popularly called _wind-eggs_.
Reckoning, then, from the shell inward, there are _six_ different
envelopes, of which one only could be detected before the descent of the
egg into the oviduct: the shell; the external layer of the membrane
lining the shell; the internal layer of same lining; the white, composed
of a thinner liquid on the outside, and a thicker and more yellowish
liquid on the inside; the bulgings, or chalaziferous membrane; and the
proper membrane.
One important part of the egg is the _air-bag_, placed at the larger
end, between the shell and its lining membrane. This is about the size
of the eye of a small bird in new-laid eggs, but is increased as much as
ten times in the process of hatching. The air bag is of such great
importance to the development of the chicken--probably by supplying it
with a limited atmosphere of oxygen--that, if the blunt end of an egg be
pierced with the point of the smallest needle, the egg cannot be
hatched.
Instead of one rudimental egg falling from the ovarium, two may be
detected, and will, of course, be inclosed in the same shell, when the
egg will be double-yolked. The eggs of a goose have, in some instances,
contained even three yolks. If the double-yolked eggs be hatched, they
will rarely produce two separate chickens, but, more commonly,
monstrosities--chickens with two heads, and the like.
The _shell_ of an egg, chemically speaking, consists chiefly of
carbonate of lime, similar to chalk, with a small quantity of phosphate
of lime, and animal mucus. When burnt, the animal matter and the
carbonic acid gas of the carbonate of lime are separated; the first
being reduced to ashes, or animal charcoal, while the second is
dissipated, leaving the decarbonized lime mixed with a little phosphate
of lime.
The _white_ of the egg is without taste or smell, of a viscid, glairy
consistence, readily dissolving in water, coagulable by acids, by
spirits of wine, and by a temperature of one hundred and sixty-five
degrees, Fahrenheit. If it has once been coagulated, it is no longer
soluble in either cold or hot water, and acquires a slight insipid
taste. It is composed of eighty parts of water, fifteen and a half parts
of albumen, and four and a half parts of mucus; besides giving traces of
soda, benzoic acid, and sulphuretted hydrogen gas. The latter, on an egg
being eaten on a silver spoon, stains the spoon of a blackish purple, by
combining with the silver, and forming sulphuret of silver.
The white of the egg is a very feeble conductor of heat, retarding its
escape; and preventing its entrance to the yolk; a providential
contrivance, not merely to prevent speedy fermentation and corruption,
but to arrest the fatal chills, which might occur in hatching, when the
mother hen leaves her eggs, from time to time, in search of food. Eels
and other fish which can live long out of water, secrete a similar
viscid substance on the surface of their bodies, furnished to them,
doubtless, for a similar purpose.
The _yolk_ has an insipid, bland, oily taste; and, when agitated with
water, forms a milky emulsion. If it is long boiled it becomes a
granular, friable solid, yielding upon expression, a yellow, insipid,
fixed oil. It consists, chemically, of water, oil, albumen, and
gelatine. In proportion to the quantity of albumen, the egg boils hard.
The weight of the eggs of the domestic fowl varies materially; in some
breeds, averaging thirty-three ounces per dozen, in others, but fourteen
and a half ounces. A fair average weight for a dozen is twenty-two and a
half ounces. Yellow, mahogany, and salmon-colored eggs are generally
richer than white ones, containing, as they do, a larger quantity of
yolk. These are generally preferred for culinary purposes; while the
latter, containing an excess of albumen, are preferred for boiling,
etc., for the table.
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