<Tr> <Td> </Td> <Td> This article needs attention from an expert in Food and drink . Please add a reason or a talk parameter to this template to explain the issue with the article . WikiProject Food and drink may be able to help recruit an expert . (February 2009) </Td> </Tr> <P> Among those animals which produce one, the yolk (also known as the vitellus) is the nutrient - bearing portion of the egg whose primary function is to supply food for the development of the embryo . Some kinds of egg contain no yolk, for example because they are laid in situations where the food supply is adequate (such as in the body of the host of a parasitoid) 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 . </P> <P> As stored food, yolks are often rich in 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 . The yolk is not living cell material like protoplasm, but largely passive material, that is to say deutoplasm . The food material and associated control structures are supplied during oogenesis . Some of the material is stored more or less in the form in which the maternal body supplied it, partly as processed by dedicated non-germ tissues in the egg, while part of the biosynthetic processing into its final form happens in the oocyte itself . </P> <P> Apart from animals, other organisms, like algae, specially in the oogamous, can also accumulate resources in their female gametes . In gymnosperms, the remains of the female gametophyte serve also as food supply, and in flowering plants, the endosperm . </P>

Does the egg yolk or egg white have protein