17. Contents Of Chicken Egg
egg, in all its complexity, is still a mystery. A highly complex
reproductive cell, it is essentially a tiny center of life.
Initial development of the embryo takes place in the blastoderm.
The albumen surrounds the yolk and protects this potential life.
It is an elastic, shock-absorbing semi-solid with a high water
content. Together, the yolk and albumen are prepared to sustain
life - the life of a growing embryo - for three weeks, in the
case of the chicken. This entire mass is surrounded by two
membranes and an external covering called the shell. The shell
provides for an exchange of gases and a mechanical means of
conserving the food and water supply within.
is formed in the mature hen by a reproductive system composed of
an ovary and oviduct. Most females have two functional ovaries,
but chickens and most other birds have only one ovary and one
oviduct. In this oviduct, all parts of the egg, except the yolk,
are formed. It is divided into five distinct regions: (1)
infundibulum or funnel, (2) magnum, (3) isthmus, (4) uterus or
shell gland, and (5) vagina.
yolk is formed in the follicular sac by the deposition of
continuous layers of yolk material. Ninety-nine percent of the
yolk material is formed within the 7-9 days before the laying of
the egg. When the yolk matures, the follicular sac ruptures or
splits along a line with few, of any, blood vessels. If any
blood vessels cross the stigma, a small drop of blood may be
deposited on the yolk as it is released from the follicle. This
causes most blood spots in eggs. After the yolk is released from
the follicle, it is kept intact by the vitelline membrane
surrounding it. The release of the yolk from the ovary is called
its release from the follicle, the yolk falls into the hen's
abdominal cavity. The infundibulum of the oviduct quickly
engulfs the yolk with its thin, funnel-like lips. The yolk
quickly enters the magnum section of the oviduct where the dense
portion of the albumen is added. The shape of the egg is largely
determined in this section.
magnum of the oviduct is divided from the isthmus by a narrow,
translucent ring without glands. The isthmus is smaller in
diameter than the magnum. It is here the two shell membranes
form. The shell membranes loosely contain the yolk and dense
white until the rest of the albumen is added in the uterus.
shell is added in the uterus or shell gland portion of the
oviduct. The shell is composed mainly of calcium carbonate. It
takes about 20 hours for the egg shell to form. If the hen lays
brown eggs, the brown pigments are added to the shell in the
last hours of shell formation.
chalazae, two cord-like structures which keep the yolk centered
in the egg, first appear in the uterus. The chalazae also
function as an axis around which the yolk can rotate and keep
the germinal disc in hatching eggs uppermost at all times.
last portion of the oviduct, the vagina, a thin, protein coating
called "bloom" is applied to the shell to keep harmful bacteria
or dust from entering the egg shell pores. The egg passes
through the oviduct small end first, but is laid large end
first. In the vagina, the egg is turned horizontally just before
laying. If the hen is disturbed on the nest, the egg may be
prematurely layed small end first. "Oviposition" is the act of
pushing the egg from the oviduct.
egg is laid, it fills the shell. As it cools, the inner portion
of the egg contracts and forms an air cell between the two shell
membranes. A high quality egg has a tiny air cell, indicating
the egg was collected soon after being layed and was stored
properly. The air cell is usually located in the large end of
the egg where the shell is most porous and air can enter easily.