The yolk (Greek λέκιθος lekithos , Latin vitellum , especially as the yolk of a chicken egg) is an accumulation of reserve substances in an egg cell . This reservoir provides the developing embryo with building material and nutrients until the organism can take in food on its own.
Initially, these memory substance (are Nährdotter ) and the egg ( formative yolk ) jointly by the vitelline surrounded (yolk skin). In general, and especially in bird eggs, this whole is referred to as yolk or egg yolk.
For the purpose of fertilization, the egg cell lies as a flat disc ( germinal disc ) directly under the yolk membrane. This consists of protein and corresponds to the zona pellucida of mammals. The flat stages of embryonic development in birds and reptiles are also called the germinal disc . In the further course of the process, embryonic cells grow around the yolk and exploit it.
Classification of eggs
Differentiation according to the arrangement of the yolk in the egg:
- isolecithal: uniform
- anisolecithal: unevenly
- centrolecithal: accumulation in the center
- telolecithal: yolk predominantly on the vegetative pole
The yolk can not only lie in the egg (endolecithal), but also in other cells outside the vitelline membrane (ectolecithal).
Finally, eggs can be classified according to their yolk content:
- oligolecithal: little (e.g. the spawn of bony fish and amphibians)
- mesolecithal: medium
- polylecithal: a lot (e.g. bird egg )