Let's describe the egg from the outside to the inside: from the cuticle on the shell to the yolk.
The cuticle covers the shell; it is a membrane invisible to the naked eye (its thickness is about 10-30 microns), formed by a particular protein (glycoprotein), whose purpose is to protect the egg from the action of microbes and bacteria. The pigments of the shell are deposited in the cuticle.
The shell is the external part of the egg, inedible, and represents approximately 10% of the components of the egg. The color of the shell varies depending on the breed of hen.
The shell is mainly made up of calcium carbonate (97-98%) (3-2% instead of magnesium carbonate). In the shell, which is porous, there are 7,000-17,000 pores that regulate air exchange and prevent bacteria from penetrating the egg while allowing humidity and carbon dioxide to exit the egg.
Adherent to the inside of the shell are two testaceous membranes (internal and external), between which, in the rounded part of the egg (obtuse pole), is the air chamber.
The testaceous membranes are semipermeable and allow the passage of gas, water and crystalloids but not of albumen. The air chamber increases in volume as the egg ages and is one of the tools for evaluating the freshness of the egg.
The chalazae appear as two twisted white cords that extend from the yolk to the two poles of the egg (obtuse and acute), along the longitudinal axis, whose purpose is to keep the yolk suspended in the center of the egg. The chalazae take on the characteristic twisted shape due to the continuous rotation to which the egg is subjected in the uterus.
The albumen (from the Latin “albus”, or white) is a viscous, semi-liquid and yellowish substance, tasteless and odorless, consisting mainly of water (87-89%). It has antimicrobial properties to protect the yolk. The albumen is the most abundant mass found in the egg, reaching up to 60% of the total weight of the egg.
The vitelline membrane is the shiny, transparent and taut membrane that encloses the yolk. When it is torn or damaged, it causes the yolk to leak out.
The yolk (from the Latin torulum, "small swelling"), or yolk, has a spherical shape of yellow-orange color and constitutes approximately 30% (30-33%) of the total weight of the egg.
It is completely surrounded by the albumen which also protects it from impacts and shocks.
The yolk always occupies the center of the egg and is suspended in the albumen by means of special cords (chalazia) that act as shock absorbers.
The germinal disc is the small circular white mass of 2-3 millimeters of white color that is found in the yolk. The germinal disc is also present in the unfertilized egg.