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.

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