One Egg, Two Yolks

How are eggs (hen’s) with two yolks produced? Can that
happen naturally?

Answer 1: Double-yolked eggs are caused by entrapment
of two yolks in one shell and the average frequency in
nature is about 1 in 1000.

This phenomenon occurs primarily due to the age of the
hen, but can also occur due to other factors such as
genetic conditions (for example, heavy laying breeds).

Young hens that have not yet developed a laying pattern
commonly produce double-yolked eggs. Each egg yolk
starts as a follicle cell in the hen’s ovary and it reaches the
funnel-shaped infundibulum.

Here is where the rooster’s sperm cells get collected and
fertilization occurs. Then the yolk reaches the magnum
where albumin layers (part of the egg white) are added.
The next part of the oviduct is the isthmus where water and
more albumins are added and the ‘egg’ gets covered by
two fibrous protein membranes.

The egg then reaches the ‘shell gland’ where it gets the
hard calcium shell. Finally, the egg is laid.

Young hens sometimes secrete two follicles in quick
succession from the ovary. When both of them are
together in the oviduct, they get ‘entrapped’ in the same
shell leading to a double-yolked egg.

Rarely, one of the follicles remains in the infundibulum too
long to be joined by the next secreted follicle leading to
double-yolked eggs.

Breakage of non-follicle tissue from the ovary is sometimes
mistaken as a yolk and gets covered in the oviduct with
albumin and shell. This leads to an egg with no yolk inside.

Double-yolked eggs do not hatch two chicks; usually both
the chicks die. There is not enough space for two chicks in
the eggshell and neither of the chicks can reach the air-
cell (seen in between the two membranes at one end of
the egg).

Answer 2: The yolk inside the egg that is formed in the
ovary is released by ovulation. A fully formed egg is laid
after passing through several stages.

The time gap between two ovulations is generally more
than 23 hours.

When there is a release of second yolk at quick
succession before the first one passed down to form a
complete egg, the double yolk egg results. In this both the
yolks are processed together and are encased in a single
shell.

This is very common in chicken, which are in high rate of
lay. These eggs are larger in size than single yolk eggs
and are edible. The highly active ovary due to high activity
of reproductive hormones in peak egg production is
responsible for this. Double yolk eggs are not suitable for
hatching.  
                                                             Hindu, 23-3-06
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