Egg yolk
An egg yolk is the part of an egg which serves as the food source for the developing embryo inside.  Prior to fertilization the yolk together with the germinal disc is a single cell. Mammalian embryos live off their yolk until they implant on the wall of the uterus.  The egg yolk is suspended in the egg white (known more formally as albumen or ovalbumin) by one or two spiral bands of tissue called the chalazae.
As a food, yolks are a major source of vitamins and minerals.  They contain all of the egg's fat and cholesterol, and almost half of the protein.
If left intact while cooking fried eggs, the yellow yolk surrounded by a flat blob of egg white creates the distinctive sunny-side up form of the food.  Mixing the two components together before frying results in the pale yellow form found in omelettes and scrambled eggs.

# Uses
- It is sometimes separated from the egg white and used in cooking (for mayonnaise, custard, hollandaise sauce, creme brulee, avgolemono, and ovos-moles).
- It is used in painting as a component of traditional egg-tempera.
- It is used in the production of egg-yolk agar plate medium, useful in testing for the presence of Clostridium perfringens.
- Egg yolk also contains an antibody called Immunoglobulin yolk or (IgY). The antibody transfers from the laying hen to the egg yolk by passive immunity to protect both embryo and hatchling from microorganism invasion.
- Egg yolk can be used to make liqueurs such as Advocaat.

# Composition of chicken egg yolk
The yolk makes up about 33% of the liquid weight of the egg; it contains approximately 60 calories, three times the caloric content of the egg white.
All of the fat soluble vitamins, (A, D, E and K) are found in the egg yolk. Egg yolks are one of the few foods naturally containing vitamin D.
The composition (by weight) of the most prevalent fatty acids in egg yolk is typically as follows:
- Unsaturated fatty acids:
Oleic acid 47 %
Linoleic acid 16 %
Palmitoleic acid 5 %
Linolenic acid 2 %
- Oleic acid 47 %
- Linoleic acid 16 %
- Palmitoleic acid 5 %
- Linolenic acid 2 %
- Saturated fatty acids:
Palmitic acid 23 %
Stearic acid 4 %
Myristic acid 1 %
- Palmitic acid 23 %
- Stearic acid 4 %
- Myristic acid 1 %
Egg yolk is a source of lecithin, an emulsifier.
A large yolk contains more than two-thirds of the recommended daily limit of 300mg of cholesterol,which is rather a lot.
The yellow color is caused by lutein and zeaxanthin, which are yellow or orange carotenoids known as xanthophylls.

# Double Egg yolk
Double Yolkers appear when ovulation occurs too rapidly, or when one yolk somehow gets "lost" and is joined by the next yolk. Double yolkers may be by a pullet whose productive cycle is not yet well synchronized. They're occasionally laid by a heavy-breed hen, often as an inherited trait.

# No yolk eggs
No-yolkers are called "dwarf", "wind"  eggs.  Such an egg is most often a pullet's first effort, produced before her laying mechanism is fully geared up.  In a mature hen, a wind egg is unlikely, but can occur if a bit of reproductive tissue breaks away, stimulating the egg producing glands to treat it like a yolk and wrap it in albumen, membranes and a shell as it travels through the egg tube.  You can tell this has occurred if, instead of a yolk, the egg contains a small particle of grayish tissue. In the old days, no yolkers were called "cock" eggs. Since they contained no yolk and therefore can't hatch, our forebears believed they were laid by roosters. This type of egg occurs in many varieties of fowl.  They have been found in chickens, both standard and bantams, guineas and Coturnix Quail (about the size of a small marble).
These are also the Eggs the turn rotten (green inside) after the hen has laid the eggs and all eggs have hatched.They are mouldy.