THE STRUCTURE OF THE EGG
To be well informed of the factors influencing the quality of the egg (both hatching egg and
consumer egg) and those of the incubation and hatching process it is necessary to know the
structure of the egg.
Below you will find a rough enumeration of the most important parts.
A. The Composition
The chemical composition of a fresh egg (approximately 58 grams) expressed in percentages (%)
is as follows:
From this summary it is clear that the yolk contains far more nutrients than the albumen. The gross
weight ratio of the different parts of the eggs are as follows:
Shell and shell membranes 5,4 grams = 9%
Albumen and chalazae 37,3 grams = 64%
Yolk 15,3 grams = 27%
Total 58,0 grams = 100%
Roughly spoken, the albumen: yolk: shell ratio is 6:3:1.
B. The eggshell
The shell consists mainly of calcium carbonate. Other elements are magnesium, phosphorus and
manganese with a small amount of protein which has a connective function.
For the construction of the eggshell vitamin D plays an important part.
The average thickness of the shell is 0.35 mm. The small end of the eggshell is thickest while on
the sides the shell is thinnest.
When the environmental temperature is high, the thickness of the shell decreases as is shown
below (egg weight 58 gram):
Temperature 19C: weight of the shell 5.6 gr. (100%)
Temperature 23C: weight of the shell 5.4 gr. ( 96%)
Temperature 32C: weight of the shell 4.9 gr. ( 88%)
In the shell are many openings, the so called pores which allow the interchange of moisture and
gasses. Roughly speaking each egg has approximately 7500 pores, most of them at the broad end,
very few at the small end.
A thin membrane, the so called cuticula, covers the whole eggshell.
This membrane can be penetrated by gasses but functions as a kind defensive mechanism to
prevent the entry of bacteria.
It is made from the sticky fluid which covers an egg freshly laid and it dissolves due to the
carbondioxide activity. In this case the shine of the egg disappears and the defensive mechanism
By mechanical means the cuticula can be injured, which results in a break down of the defensive
mechanism whereby the bacteria may penetrate the egg.
C. Egg shape
The egg shape is indicated by the shape-index, which is the ratio between egg width and egg
length multiplied by 100:
Width x 100 = shape-index
An optimum shape-index is 74 i.e. an egg (58 gr.) which is 4.2 cm wide and 5.7 cm long.
An egg with a shape index of 72 is too long.
An egg with a shape index of 76 is too round.
D. Shell membranes
Each egg has two shell membranes.
Both lie adjacent to each other and to the shell.
The inner membrane encloses the albumen.
At the broad end of the egg both membranes separate; the inner one comes away to leave an air
space (air cell)
These membranes also protect the egg against bacterial invasion.
Gasses may penetrate them.
E. Air cell
As soon as the egg has been laid, the egg will cool down, the egg contents contract whereby the
air cell is created. It is normal for the air cell to be at the broad end.
There are many more pores per square centimeter here than anywhere else on the surface of the
egg. Most of the time the air cell is larger in winter than is summer (lower temperature). When egg
get old the egg contents decrease, due to water evaporation, in which case the air cell becomes
F. The albumen of the egg.
The albumen, or white, is made up of three distinct layers. They are: outer thin white, thick white
and inner thin white, which encloses the yolk. As the egg gets older the thick white is transformed
into thin white.
In the centre of the egg on both sides of the yolk lengthwise, one finds two coils of thick white, the
They arise from the thick white layer at each end and terminate in the calciferous layer, a very
thin film of dense white that surrounds the, vitelline membrane which encloses the yolk. These
chalazae hold the yolk, which has a lower gravity weight than the albumen, in the central position.
G. The yolk
The yolk is surrounded by the vitelline membrane and consists mainly of two parts: the yellow yolk
and the white yolk, which lays inside the yellow. The gravity weight of the white yolk is lower than
the yellow one.
The white yolk runs out into the yellow yolk on which end one finds the blastodisc (female germ
The blastodisc is always on the top of the yolk. If the female cell has been fertilized by the male
germ cell the blastodisc is a little bit larger than it is in the infertile egg (3 to 4 mm). At temperatures
higher than 27 (80 F) the embryonic development (if the egg is fertile) begins.