Yolk
The yolk is the yellow center of an egg. It is only one cell. It is commonly seen in fried eggs. When it is mixed with the whites, it makes scrambled eggs. It is sometimes used in cooking. It is much higher in calories than the egg white. It has fat-soluble vitamins in it.[1] Some eggs have two egg yolks in them.
Yolk Media
Diagram of a fish egg; the yolk is the area which is marked 'C'
Three similarly sized eggs in a hot frying pan. Each of the two yolks in the double-yolked eggs are smaller than typical for that size of egg.
A chicken egg frying with an extremely thick red yolk. A normal-coloured yolk can also be seen, having been accidentally burst during the frying process.
A pregnant Caridina multidentata in captivity, with oval-shaped eggs rich in yolk.
Comparison of an egg and an egg with a double-yolk (closed) 
Comparison of an egg and an egg with a double-yolk (opened) 
Double-yolk egg - Opened
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References
- ↑ Réhault-Godbert, Sophie. The Golden Egg: Nutritional Value, Bioactivities, and Emerging Benefits for Human Health. Nutrients 11 (3) (2019-03-22). p. 684. doi:10.3390/nu11030684.