Before proceeding to inquire into the interior composition of the egg,
we will consider the exterior covering, or the shell--the physical and
chemical structure of which is exceedingly interesting and wonderful.
The white, fragile cortex called the shell, composed of mineral matter,
is not the tight, compact covering which it appears to be; for it is
everywhere perforated with a multitude of holes too small to be
discerned by the naked eye, but which, with the aid of a microscope, are
distinctly revealed. Under the microscope the shell appears like a
sieve, or it more closely resembles the white perforated paper sold by
stationers. Through these holes there is constant evaporation going on,
so that an egg, from the day that it is dropped by the hen, to the
moment when it is consumed, is losing weight and diminishing in volume.
This process goes on much more rapidly in hot weather than in cold, and
consequently perfect eggs are not so readily procured in summer as in
winter. If, by any means, we stop this evaporating process, the egg
remains sound and good for a great length of time. Covering the shell
with an impervious coat of varnish, or with mutton suet or lard, aids
greatly in their preservation. The substance used to stop transpiration
must not be soluble in watery fluids, or liable to be removed. By
chemical agencies, that is, by actually filling up the little holes in
the shell by lime placed in solution (the solution holding the proper
chemical substances to form an impervious coating of carbonate of lime
over the entire surface), we have preserved eggs for months, and even
years, in a sweet condition. Not long ago, eggs broken in a laboratory
in Boston were found to be quite fresh, which, according to the
memorandum made upon the vessel, were placed in the solution four years
ago.
[Illustration]
The shell of the egg is lined upon its interior everywhere with a very
thin but pretty tough membrane, which, dividing at or very near the
obtuse end, forms a small bag which is filled with air. In new-laid eggs
this follicle appears very little, but it becomes larger when the egg is
kept. In breaking an egg, this membrane is removed with the shell to
which it adheres, and therefore is regarded a part of it, which it is
not.
The shell proper is made up mostly of earthy materials, of which
ninety-seven per cent is carbonate of lime. The remainder is composed of
two per cent of animal matter and one of phosphate of lime and magnesia.
Carbonate of lime is the same material of which our marble quarries and
chalk beds are composed: it is lime, or oxide of calcium, combined with
carbonic acid, and is a hard, insoluble mineral substance, which does
not appear to form any portion of the food of fowls. Now, where does the
hen procure this substance with which to form the shell? If we confine
fowls in a room and feed them with any of the cereal grains, excluding
all sand, dust or earthy matter, they will go on for a time and lay
eggs, each one having a perfect shell made up of the same calcareous
elements. Vanvuelin, the distinguished chemist, shut up a hen ten days
and fed her exclusively upon oats, of which she consumed 7,474 grains in
weight. During this time four eggs were laid, the shells of which
weighed nearly 409 grains: of this amount 276 grains was carbonate of
lime, 17-1/2 phosphate of lime, and 10 gluten. But there is only a
little carbonate of lime in oats, and from whence could this 409 grains
of the rocky material have been derived? The answer to this question
opens up some of the most curious and wonderful facts connected with
animal chemistry, and affords glimpses of many of the operations of
organic life, which, to the common mind, seem in the highest degree
paradoxical and perplexing. The body of a bird, like that of a man, is
but a piece of chemical apparatus made capable of transforming hard and
fixed substances into others of a very unlike nature. In oats there is
contained phosphate of lime, with an abundance of silica; and the
stomach and assimilating organs of the bird are made capable of
decomposing or rending asunder the lime salts and forming with the
silica a silicate of lime.
This new body is itself made to undergo decomposition, and the base is
combined with carbonic acid, forming carbonate of lime. The carbonic
acid is probably derived from the atmosphere, or more directly, perhaps,
from the blood. These chemical changes among hard, inorganic bodies are
certainly wonderful when we reflect that they are brought about in the
delicate organs of a comparatively feeble bird, under the influence of
animal heat and the vital forces. They embrace a series of decomposing
and recomposing operations which it is difficult to imitate in the
laboratory.
In the experiment to which allusion has been made, the amount of earthy
material found in the eggs and the excrement of the hen exceeded that
contained in the food she consumed. This seems paradoxical, and can only
be explained upon the ground that birds as well as animals have the
power, in times of exigency, of drawing upon their own bodies for
material which is required to perform necessary functions.
The shell of an ordinary hen's egg weighs about one hundred and six
grains, that is, the inorganic portion of it; and if a bird lays one
hundred eggs in a year, she produces about twenty-two ounces of nearly
pure carbonate of lime in that period of time, which would afford chalk
enough to meet the wants of a farmer, or perhaps even of a house
carpenter of moderate business, for a twelvemonth.
If a farmer has a flock of one hundred hens, they produce in egg shells,
about one hundred and thirty-seven pounds of chalk annually; and yet not
a pound of the substance, or perhaps not even an ounce, exists around
the farm-house within the circuit of their feeding grounds. This is a
source of lime production not usually recognized by farmers or hen
fanciers, and it is by no means insignificant. The materials of the
manufacture are found in the food consumed, and in the sand, pebble
stones, brick-dust, bits of bones, etc., which hens and other birds are
continually picking from the earth.
The instinct is keen for these apparently innutritious and refractory
substances, and they are devoured with as eager a relish as the cereal
grains or insects. If hens are confined to barns or outbuildings, it is
obvious that the egg-producing machinery cannot be kept long in action,
unless the materials for the shell are produced in ample abundance.
Within the shell the animal portion of the egg is found; which consists
of a viscous, colorless liquid called albumen, or the _white_, and a
yellow globular mass called the vitellus, or _yolk_. The white of the
egg consists of two parts, each of which is enveloped in distinct
membranes. The outer bag of albumen, next the shell, is quite a thin,
watery body, while the next which invests the yolk, is heavy and thick.
But few housekeepers who break eggs ever distinguish between the _two
whites_, or know of their existence even.
Each has its appropriate office to fulfil during the process of
incubation or hatching; and one acts in the mysterious process as
important a part as the other. If we remove this glairy fluid from the
shell and place it in a glass, and plunge into it a strip of reddened
litmus paper, a blue tinge is immediately produced, which indicates the
presence of an alkali. The alkali is soda in a free condition, and its
presence is of the highest consequence, for without it the liquid would
be _insoluble_. A portion of the white of an egg, when diluted with
water, and a few drops of vinegar or acetic acid added to it, undergoes
a rapid change. The liquid becomes cloudy and flocculent, and small bits
of shreddy matter fall to the bottom of the vessel. This is pure
albumen, made so by removing the soda held in combination by the use of
the acid. A pinch of soda added to the solid precipitate redissolves it,
and it is again liquid. There is another way by which the albumen is
rendered solid: and that is by the application of heat. Eggs placed in
boiling hot water pass from the soluble to the insoluble state quite
rapidly, or, in other words, the albumen both of the white and yolk
becomes "coagulated."
No contrast can be greater than that between a boiled and unboiled egg.
Not only is it changed physically, but there is a change in chemical
properties, and yet no chemist can tell in what the change consists. It
is true, the water extracts a little alkali, and a trace of sulphide of
sodium; but the abstraction of these bodies is hardly sufficient to
account for the change in question.
The hardening of the albumen of egg by heat constitutes the cooking
process, and this deserves a moment's consideration.
Great as is the physical difference between a fully cooked and an
uncooked egg, it is no less remarkable in the degree of digestibility
conferred upon it by the process. Uncooked, it passes by the most simple
processes of assimilation from the digestive to the nutritive and
circulatory organs, and is at once employed in nourishing or sustaining
the bodily functions. Unduly cooked, the egg resists the action of the
gastric juices for a long time, and becomes unsuited to the stomachs of
the weak and dyspeptic. A raw or soft-boiled egg is of all varieties of
food the most nourishing and concentrated; a hard-boiled egg is apt to
trouble the digestion of the strong and healthful, and its nutrient
properties are sensibly impaired. The yolk contains water and albumen,
but associated with these is quite a large number of mineral and other
substances which render it very complex in composition. The bright
yellow color is due to a peculiar fat or oil, which is capable of
reflecting the yellow rays of light, and this oil holds the sulphur and
phosphorous which abound in the egg. If the yolk be removed and dried,
and the yellow oil separated, it will be found to form two-thirds of the
substance. The whole weight in its natural state is about three hundred
grains, of which three-fifths is water; of the white more than three
quarters is water.
The yolk and albumen of a fecundated egg remains as sweet and free from
corruption during the whole time of incubation as it is in new-laid
eggs, and there is but little loss of water; whereas an unfecundated egg
passes rapidly into putrefactive decay and perishes.
Any one who eats three or four eggs at breakfast consumes that number of
embryo chicks.
All the materials which enter into the legs, bones, feathers, bill,
etc., of the new-born chick, exist in the egg, as nothing is derived
from outside. The little creature which has just pecked its way out of
its calcareous prison-house, has lime and phosphorus in his bones,
sulphur in his feathers, iron, potash, soda and magnesia in his blood,
all of which mineral constituents come from the egg, and are taken into
the stomach when it is eaten as food.
The valuable or important salts are contained in the yolk, and hence
this portion of the egg is the most useful in some forms of disease. A
weakly person, in whom nerve force is deficient and the blood
impoverished, may take the yolks of eggs with advantage. The iron
phosphoric compounds are in a condition to be readily assimilated, and
although homoeopathic in quantity, nevertheless exert a marked
influence upon the system. The yolks of eggs, containing as they do less
albumen, are not so injuriously affected by heat as the white, and a
hard-boiled yolk may be usually eaten by invalids without inconvenience.
The composition of a fresh egg, exclusive of the shell, may be presented
as follows:
Water 74.0 parts.
Albumen 14.0 parts.
Oil or fat 10.5 parts.
Mineral Salts 1.5 parts.
------
100
The whole usually weighs about a thousand grains, of which the shell
makes a tenth part.
The chick-making materials, exclusive of water, form only one-quarter of
the weight of the liquid contents, or only about two hundred grains.
This seems to be a small beginning upon which to rear a full-grown
rooster. The bulk or quantity as found in hens' eggs, and indeed in the
eggs of all birds, is wonderfully disproportionate to the size of the
mother bird. The laying of eggs must be regarded as a particularly
exhausting process, and yet fowls will keep it up for a long time and
not lose much in flesh. We have known a hen of the game variety which
has laid twenty-two eggs in twenty-two consecutive days, and they
average in weight one thousand grains each. This gives in amount
twenty-two thousand grains, or rather more than three pounds
avoirdupois, of which about two and a quarter pounds is water. The dozen
or more ounces of rich, nutritive material, parted with in twenty-two
days, would seem to be a prodigious draught upon the small physical
structure of the bird, but there were no indications of exhaustion.
Whilst it is true that the quickening of an egg which results in the
birth of a chick, is no more marvellous a process or result than the
embryotic development of any creature endowed with the mysterious
principle of life, yet there are some circumstances connected with it
which make it a matter of greater perplexity and wonder. Here is an oval
white body consisting of a calcareous shell, within which there are some
semi-fluid substances, consisting mainly of albumen and water, without
any signs of life. In fact, there is no life; it is simply a mass of
dead, inanimate matter. Talk as much as we will about the germinal
principles involved in the structure of the egg, we are totally unable
to recognize it, or form any conception of its nature.
There is no evidence of the presence of any germ or principle of life
whatever. The egg left to itself decays like other organized substances,
but with our assistance in simply transferring it to a place where the
temperature is in a certain uniform condition, in a few weeks, the
albumen, water, oil and mineral salts are transferred into a living
chick, which thrusts its little beak through the shell, and in ten
minutes is running about, almost able to take care of itself.
Here is the development of life apparently without the agency of the
mother, and what a marvel! The chemist may place together in a body in a
warm place, just such elements or substances: he may carefully weigh the
water, the albumen, the phosphotic compounds, the sulphur, the iron,
soda, etc., and construct a very accurate egg mixture, but out of it all
there will never come a living chick. In this, we obtain some idea how
little we actually know about life, how dark is the region where the
life principle begins, or where the vital forces originate. The
indefatigable man of science has pushed his inquiries close up to the
boundary between the inanimate and the animate; but he has never been
able to obtain the least glimpse of anything upon the _life_ side of the
line. However great maybe our curiosity, our skill or knowledge in this
state of existence, there is not the least probability that we shall
ever be able to endow matter with life, or know much more than we do at
present of its origin or nature.
* * * * *
AUNTIE, to a little four-year-old who is resting his head on the
table--"Ah, Louis, you are sleepy; you will have to go to bed." "Oh, no,
auntie, I aren't sleepy; but my head is loose, so I laid it down here."
* * * * *