Shell: It forms the protective covering of the inner contents of the egg along with the two membranes. Shell is made up of protein polysaccharide complex and calcium carbonate. An egg shell is bride and easily breaks. Some shells are glossy others dull. Some may be smooth and others may be rough.
It is previous and contains thousands of small holes which allow gases to pass in and out of the egg for the developing embryo. The small holes are covered with a thin layer of gelatinous material mucoprotein called cuticle or bloom. The cuticle seals off the pores of the shell to some extent and helps avoid an excessive evaporation from the inner contents of the egg, It also restricts the entry of micro-organisms into the egg and thus protects the inner contents from various infections. The cuticle is soluble in water and easily removed by washing which results in hastening the deterioration of egg quality.
It is primarily calcium carbonate in an organic matrix. The composition of egg shell is given in Table. This portion of the egg is inedible.
Shell membranes: Within the shell are inner and outer membranes that also protect the quality of the egg. Both the membranes are porous and composed of fibres. The outer membrane which is thicker (48µm) than the inner one (22µm) is firmly attached to the shell. The outer membrane has six layers of fibres, whereas, the inner one has three layers. The inner membrane is attached to the outer and the two membranes are loosely attached to one place usually at the broad end of the egg. The membranes are composed of protein and polysaccharide.
Eggs contain little or no air cells when they are laid. After being laid because of the lower temperature of the outer surroundings of the egg than when it was in the hen’s body, there is contraction of the inner contents of the egg. This results in air being drawn into the shell resulting in a small air cell formation between the shell membranes usually at the large end of the egg.
The condition of the shell and the membrane influence moisture and carbon dioxide, breaking strength and susceptibility to microbial invasion.
Egg white: The white of the egg consists of three layers, two areas of thin white encompassing one area of thick white. Some hens secrete a higher ratio of thick to thin white than do others. Storage conditions also affect the thickness of the albumin and the ratio of thick to thin egg white.
Egg yolk: The yolk of the egg is enclosed in a sac called the vitelline membrane. Immediately adjacent to the vitelline membrane, the thin membrane that surrounds the egg yolk, is chalaziferous or inner layer of firm white. This chalaziferous layer gives strength to the vitelline membrane and extends into the chalazae. The chalazae appear as two small twisted ropes of thickened white, one on each end of the yolk and anchor the yolks in the centre of the egg. Chalazae appear to have almost the same molecular structure as ovomucin.
The yolk carries the indistinct germinal disc or germ spot which under suitable conditions develops into a chick. Beneath the germ spot extends a white part called latebra. The yolk itself is layered into sections of white and yellow yolk but they are not readily distinguishable.
In infertile eggs, the female reproduction nucleus is not fertilized by union with the sperm of the cock. These are incapable of producing chicks and therefore called lifeless or vegetarian eggs. Infertile eggs are produced when a male bird is not kept with the laying hens.
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