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The hen's body temperature is 105° F to 106° F. When the hen sits on the eggs,
she heats the eggs to 100° F to 101° F. The hen turns the eggs on a regular basis by
using her beak to scoop under the egg and roll it toward her. The humidity comes from
the environment, the hen's body, and any moisture she transfers back to the nest on
her feathers. Brooding hens often leave their nests to feed at dawn or dusk when the
dew is present on the grass.
The development of the chick begins in the single cell formed by the union of
two parental cells, egg and sperm, in the process known as fertilization. In birds,
fertilization occurs about 24 hours before the egg is laid.
The newly formed single cell begins to divide into 2, then 4, 8, 16, 32 and so on.
At the time of laying, hundreds of cells are grouped in a small, whitish spot (the
germinal disc) that is easily seen on the surface of the yolk.
When the egg is laid and cools, division of the cells ceases. After the egg is laid,
cooling the egg does not result in the death of the embryo. It may resume its
development after several days of rest if it is again heated by the hen or in an
incubator.
Functions of the Embryonic Membranes
Special temporary organs or embryonic membranes form within the egg, both
to protect the embryo and to provide for its nutrition, respiration, and excretion.
These organs include the yolk sac, amnion, and allantois.
The yolk sac supplies food material to the embryo. The amnion, by enclosing the
embryo, provides protection. The allantois serves as a respiratory organ, gets minerals
from the shell, and handles waste. These temporary organs function within the egg
until the time of hatching.
Hatching
Several changes take place during the 18th and 21st days. The chick draws
what remains of the yolk into its body and gets plenty of nourishment from it. Thus,
the chick really doesn't need to be fed for a few days after it hatches.
The chick's head is under its right wing with the tip of the beak pointed at the
air shell. The large neck muscle contracts and forces the egg tooth through the air
cell, and the chick takes its first breath. This is referred to as internal pipping. At
this time, you may hear the chick peeping inside the shell.
On the twenty-first day of incubation the chick finishes its escape from the
shell. The egg tooth makes the initial break in the shell. This is referred to as
external pipping.
The hatching process can last from 4 to 12 hours before the chick breaks free
from the shell. The chick, upon freeing itself from the shell, is wet and very tired. For
the next several hours it will lie still and rest. A few hours later the chick, now dry
and fluffy, will become extremely active and the egg tooth will dry and fall off.