In poultry, the term egg can be used for the whole set of shell, egg yolk; or be used in the strictly embryological sense: fertilized egg or zygote.
In chickens, oocyte II is released from the ovaries and enters the oviduct, where it can be fertilized by sperm. Embryonic development begins in the oviduct and is completed outside the bird's body.
The developing egg, while still in the anterior portion of the oviduct, is surrounded by a dense albumen, secreted by glandular cells from the wall of the organ that serves as food for the embryo. In this region, besides glands, there are spiral folds that determine the rotation of the egg when it passes through the oviduct. calaza.
1 - Fork Membrane
4 - Calaza
6 - Clara
7- Vitelline membrane
9 - Germen
13 - Calaza
14 - Inner Tube
15 - Shell or peel
Subsequently, in the oviduct, an albumin is added but less consistent. An elastic film is added around the whole white. In the terminal portion, the porous limestone bark, formed by calcium and magnesium carbonates and phosphates, is secreted.
The residence time of the developing embryo within the oviduct is about 24 hours for chickens. When the egg is laid, the embryo is already at the end of the segmentation. After an incubation period of 21 days at 37.5ºC hatching occurs.
The egg yolk may correspond to oocyte II when there is no fertilization, or to the embryo when fertilization occurs.