Egg Yolks Dried by Country

The yolk in an egg is material of which the primary function is to supply food for the development of the animal embryo. Some kinds of eggs contain no yolk, for example because they are laid in situations where the food supply is adequate  or because the embryo develops in the parent's body, which supplies the food, sometimes through a placenta.

Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic those in which the embryo is supplied by yolk are said to be lecithotrophic. In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fishes and invertebrates, the yolk material is not in a special organ,but inside the ovum.

Yolks, being mainly stored food, tend to be very concentrated, and in particular tend to be rich in nutrients such as vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in controlling the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization. Yolk is not living cell material like protoplasm, but largely passive material, that is to say deutoplasm, in this case functioning mainly as food for the embryo.