outer eggshell is made almost entirely of calcium carbonate (CaCO3) and is covered
with as many as 17,000 tiny pores. It is a semipermeable membrane, which allows
air and moisture to pass through its pores. The shell also has a thin outermost
coating called the bloom or cuticle that helps keep out bacteria and dust (see
Outer shell membrane
3. Inner shell membrane
two membranes -- outer and inner -- are just inside the shell surrounding the
albumen (white). The two membranes provide an efficient defense against bacterial
invasion and are made partly of keratin. The
outer membrane sticks to the egg shell while the inner membrane sticks to the
albumen. When an egg is first laid, it is warm. As it cools, the contents contract
and the inner shell membrane separates from the outer shell membrane to form the
air cell (see 14 below).
Chalaza -- are twisted in opposite directions and serve to keep the yolk centered.
The more prominent the chalazae, the fresher the egg.
Exterior albumen (outer thin albumen) -- The
outer thin albumen is a narrow fluid layer next to the shell membrane.
Middle albumen (inner thick albumen) --The
inner thick white (chalaziferous layer) is a dense, matted, fibrous capsule of
albumen around the vitelline membrane of the yolk. The matted fibrous capsule
terminates on each end in the chalazae, which are twisted in opposite directions
and serve to keep the yolk centered. This part of the egg is a excellent
source of riboflavin and protein. In high-quality eggs, the inner thick albumen
stands higher and spreads less than thin white. In low-quality eggs, it appears
Vitelline membrane -- The
clear casing that encloses the egg yolk. When
an egg is said to be "mottled", the yolk surface is covered with many pale spots
or blotches. The strength and integrity of the vitelline membrane are very important
in preventing egg yolk mottling.
Nucleus of pander -- a plug of whitish yolk, with no particular significance
for development and whose function is purely a nutritive one, like the rest of
the yolk. (See: Int. Schmitt S., (2005) J. Dev. Biol. 49: 1-8).
Germinal disk (blastoderm) -- a small, circular, white spot (2-3 mm across)
on the surface of the yolk; it is where the sperm enters the egg. The nucleus
of the egg is in the blastodisc. The embryo develops from this disk, and gradually
sends blood vessels into the yolk to use it for nutrition as the embryo develops.
yolk -- a major
source of vitamins, minerals, almost half of the protein, and all of the fat and
cholesterol. The yolk
contains less water and more protein than the white, some fat, and most of the
vitamins and minerals of the egg. These include iron, vitamin A, vitamin D, phosphorus,
calcium, thiamine, and riboflavin. The yolk is also a source of lecithin, an effective
emulsifier. Yolk color ranges from just a hint of yellow to a magnificent deep
orange, according to the feed and breed of the hen.
White yolk -- Also known as, the latebra is an area of white yolk located
in the center of the yolk. It is lower in fat and therefore stands out as a bright
white area in many Magnetic Resonance Images. The specific function of the latebra
is uncertain but it may act as a central structure around which the additional
layers of the yolk are formed.
Internal albumen (Chalaziferous albumen)-- The inner thick white (chalaziferous
layer) is a dense, matted, fibrous capsule of albumen around the vitelline membrane
of the yolk. The matted fibrous capsule terminates on each end in the chalazae,
which are twisted in opposite directions and serve to keep the yolk centered.
chalazae, which are twisted in opposite directions and serve to keep the yolk
centered. The more prominent the chalazae, the fresher the egg. chalazae, which
are twisted in opposite directions and serve to keep the yolk centered.
Air cell -- An air space forms when the contents of the egg cool and contract
after the egg is laid. The air cell usually rests between the outer and inner
membranes at the egg’s larger end. As
the egg ages, moisture and carbon dioxide leave through the pores of the shell,
air enters to replace them and the air cell becomes larger.
Cuticle or bloom -- The shell is produced by the shell gland (uterus) of the
oviduct, and has an outer coating, the bloom or cuticle. The cuticle somewhat
seals the pores and is useful in reducing moisture losses and in preventing bacterial
penetration of the egg shell. Most of cuticle is removed from table eggs when
they are mechanically washed.