First I need to ask you...I need to ask you first… How does a chicken leave the hen-hen house?
Obviously, though the eggs-it.
Okay, okay… Enough yolking around. Lets get cracking!
To be honest, as a veterinarian I expected I’d be able to answer the long time question of “what came first: the chicken or the egg”. But, all I have for you is an explanation on the two yolk fiasco!
First, there is something you need to know about eggs. An egg does not need to be fertilized to be formed! An egg is a biproduct of ovulation; not ovulation and fertilization.
So, lets go back to sexual education 101: poultry edition.
The reproductive tract of the chicken is made up of two parts: the ovary and the oviduct. The ovary grows the yolk and once it reaches the right size, it is ovulated (or released) into the oviduct. This is where fertilization would take place if sexual coitus did occur. If no semen is present, the yolk is transferred through the rest of the oviduct where it gains the egg white and shell. The egg then spends the rest of its time developing in the shell gland, where the shell and shell pigments are added. It takes about 25-26 hours for a yolk to develop into a complete egg. Once complete, the egg travels into the vagina and is laid through the cloaca. Typically, 30-75 minutes after a hen lays an egg, her ovary will release the next yolk.
NOW TO ANSWER THE QUESTION: how does a single egg end up with two yolks?
You get a DOUBLE YOLK when the ovary is a little too overly excited and ovulates another a second yolk shortly after! The two yolks will then move together through the oviduct, and get laid as a single egg!
This actually isn’t as rare as people think. Apparently, it happens in 1 in 1000 eggs and happens more frequently in younger laying hens.