Experts say that a hen’s reproductive system is designed to release yolks at regular intervals. And when the yolk starts as an immature egg cell in the ovary, it grows and layers of nutrients build up around it, forming the familiar yellow yolk. When the yolk is mature, it is released from the ovary in a process called ovulation. It then travels through the oviduct, where the egg white, membranes and shell are gradually added. And in normal situations, one yolk enters the oviduct at a time (30 minutes after laying an egg). But in rare situations, two yolks are released close together and travel through the reproductive tract together. The hen’s body then surrounds both with egg white and eventually forms a single shell around them, resulting in a double-yolk egg.
