In theory, eggs with two yolks could generate two chicks, but what is seen in practice is that neither survives. This happens because the air chamber inside the egg – essential for completing the chicks’ lung cycle – is too small for both of them. “Since this chamber only has room for the head of one of them, in 99% of cases both end up dying”, says zootechnician Edvaldo Garcia, from Unesp. Such an air chamber is a space between the membrane that lines the egg shell internally and another membrane that surrounds the chick. In the final stage of its development, the chick uses its beak to pierce the film that protects it and starts breathing the oxygen in the chamber. Double gestation is the result of an irregularity in the hens’ hormonal cycle. The reproductive organ of birds is shaped like a funnel. The yolk, where the egg is, is released from the ovary and falls into the “funnel”, where it meets the white. When hormonal irregularity occurs, two yolks are released from the ovary.
In the final part of the reproductive organ, yolk and white are packed by the shell, thus forming the egg.