Physical Composition Of Eggs
Egg consists of 3 major parts:
1.Shell and shell membranes forms 11% of whole shell egg.
2.Albumen forms 58% of whole shell egg
3.Yolk forms 31% of whole shell egg
a.Egg shell consists of:
2.Spongy or calcareous layer
3.Mammillary layer or matrix and
The pores are funnel shaped, distributed at right angles to the shell surface and form connecting passages between the shell membrane and cuticle. The average number of pores is about 8,000 per egg, which are distributed unevenly over the shell surface; with more number of pores towards the broad end than at the narrow end.
b.Shell membrane consists of:
2.Outer shell membrane and
3.Inner shell membrane.
The inner and outer shell membrane, measures 40-70µ and 10-20µ in thickness respectively and the air cell is situated in between the two membranes, usually on the broader end.
c.Albumen consists of 4 layers, namely:
1.Chalaziferous or inner thick white, which forms 3% of total albumen
2.Inner thin albumen: 17% of the total albumen
3.Outer thick firm or dense albumen: 57% of total albumen and
4.Outer thin albumen: 23% of total albumen.
The chalaziferous layer is very close to the yolk immediately surrounding the vitelline membrane of the yolk. This is very firm (thick) albumen; it twists into two chords on either sides of the yolk called chalazae, which are formed due to rotational movement of the egg in the oviduct.
Functions of chalazae
1.Chalazae will hold the yolk firmly in its central position and thus serves as an anchor for yolk.
2.The chalazae are believed to contain a protein called lysozyme, which possess antimicrobial properties and helps to prevent the microbial spoilage of the egg. Ageing, improper storage and microbial spoilage makes thick albumen watery.
Yolk consists of
1.Concentric layers of dark and light yolk material, due to differences in the deposition of carotenoid pigments during day and night.
2.Latebra is the centre of the yolk, which is a small, nearly circular core of light coloured fluid, which does not completely harden on boiling.
3.Neck of latebra is the thin column of light coloured material connecting the latebra to the nucleus of Pander and germinal disc.
4.Nucleus of pander is a cup-shaped structure, which is an extension of the neck of latebra, connecting the base of the germinal disc.
5.The "germinal disc" or "germ spot" is the reproductive nucleus. It is a light coloured spot located on the surface of the yolk, just under the vitelline membrane. In an infertile egg it is unicellular (ovum) and contains haploid number of chromosomes, called "Blastodisc." It is circular in shape, with a diameter of about 3.5 mm and with vacuoles in it. Whereas in a fertile egg, it is a multi-cellular structure having diploid number of chromosomes called "Blastoderm." It is oval in shape, with an average diameter of about 4.5 mm and with no vacuoles in it.
6.The "Vitelline membrane" is a semi-permeable elastic membrane, surrounding the yolk, separating the yolk material from the albumen.