Structure of duck eggs: duck eggs are composed of eggshell, shell membrane, air chamber, protein, egg yolk, frenulum, ovule or ovule disc, etc.
1.Eggshell:
It is the outermost layer of calcareous hard shell of poultry eggs. The thickness varies with the variety (line), nutrition level, season, reproduction or genetic factors, generally between 0.26 and 0.38 mm. The thickness of the eggshell varies slightly with different parts, and the sharp end of the egg is slightly thicker than the blunt end. The outer layer of the eggshell has a shell glue film, and the eggshell is composed of a sponge layer and a nipple.
（1） The shell glue film, which is close to the outermost layer of colloid film of the egg shell, can seal the pores of the egg shell, limit the evaporation of water in the egg, and prevent pathogenic microorganisms from invading the egg. It has a certain protective effect.
（2） Sponge layer is a hard layer on the papilla layer deposited by crystalline minerals, which limits the evaporation of water in the egg and prevents pathogenic microorganisms from invading the egg, playing a certain protective role.
（3） The papilla layer, which accounts for about 1/3 of the thickness of the eggshell, is composed of numerous small round nipples. It is the basis of the sponge layer and the main component of the egg with a certain strength. The egg shell is densely covered with many pores, with different sizes and diameters of 4~40 microns. The pores are important channels for material metabolism and gas exchange during embryonic development during incubation, and the number of pores at the blunt end is the largest.
2. Shell membrane:
The shell membrane is divided into inner and outer layers, and the inner layer surrounds the protein, which is called protein membrane or inner shell membrane. The outer shell is close to the inner surface of the egg shell, which is called shell membrane. The inner shell membrane is thin, about 0.05 mm thick, and the shell membrane is three times of the inner shell membrane. There are pores on the inner shell membrane. The texture of the reticulated fiber structure on the outer shell membrane is rough, and the pores formed are large, which can be passed by microorganisms. The reticulated fiber structure texture of the inner shell membrane is tight and fine, and the pores are small, which can play a certain barrier role to the outside microorganisms.
3. Air chamber:
There is no air chamber in the oviduct of the female duck. When the egg is laid outside, because the outside temperature is lower than the rest temperature, the contents of the egg contract, which separates the inner and outer shell membrane of the blunt end (big head) of the egg to form a gap, called the air chamber. The longer the egg is kept, the more water is lost in the egg, and the air chamber increases. Therefore, the freshness of eggs can be identified according to the size of the air chamber.
4. Protein:
Protein is a white viscous transparent semi-fluid substance. It is composed of four layers of inner concentrated protein, inner diluted protein, outer concentrated protein and outer diluted protein. The inner concentrated protein is close to the surface of egg yolk, accounting for 2.7% of the whole protein; The outer layer is inner thin protein, accounting for 17.3%; The second layer is exodense protein, accounting for 57.0%; The outer thin protein is the outermost layer, accounting for about 23%. With the extension of storage time, under the action of enzymes in the protein, the thick protein gradually thins and the thin protein increases. Therefore, the viscosity of the protein can also be used to determine the freshness.
5. Egg yolk:
Egg yolk is an opaque yellow semi-fluid material, surrounded by yolk membrane, and cut the cooked yolk. It can be seen that the yolk is arranged in concentric circles by alternating yellow and white yolks. The formation of deep and shallow yolk is caused by the different metabolism of poultry day and night. The more obvious the deep and shallow yolk layer is for poultry with high lutein and carotenoid content in diet or grazing. There are 6 layers of deep and shallow yolk layer for eggs laid by female birds in continuous laying. When the number of eggs laid is less, the layer of yolk increases.
6. Tie-up:
The tether is a tie formed by the egg yolk twisted in the front and back of the egg yolk when the egg yolk rotates forward during the formation of the egg in the fallopian tube. It plays a role in fixing the position of the egg yolk, making the egg yolk always at the center of the egg and not adhering to the shell membrane, preventing the early embryo from adhering to the shell membrane during the incubation process, and ensuring the normal growth and development of the embryo.
7.Ovule or blastodisc
There is a small white dot on the surface of egg yolk called ovule. After fertilization of the egg, the fertilized egg will undergo cleavage and form a central transparent and dark embryo disc, which is slightly larger than the embryo bead. Because the specific gravity of ovule or ovule disc is lighter than that of egg yolk, it always floats on the surface of egg yolk
