The vitellus constitutes the energy reserves used by the Embryon S during the embryonic development. It is produced by the maternal organization and accumulates in the Ovocyte during the Ovogenèse. In fact the greasy substances produce it at the Insecte S and the Foie at the Vertébré S then conveyed by the Hémolymphe (insects) or by blood (vertebrate) towards the ovaries. At the vertebrate ones, in fact the estrogens start in the liver the production of vitellus in the form of a lipoprotein: the vitellogénine. At pleurodèles (Amphibians which does not lose their tail), one finds the vitellus in the form of plates made up as a majority of phosvitine and lipovitelline.

The most known vitellus is the egg yolk. The vitellus is primarily composed of lipidic reserves (lipoproteins).

The ovocytes are classified according to their wealth of vitellus:

• the ovocytes alécithes (without vitellus - example: Mammalian S placental)
• the ovocytes oligolécithes (with few vitellus - example: echinodermatous S)
• the ovocytes hétérolécithes (with much of vitellus distributed in a gradient - example: the Amphibian S)
• the ovocytes télolécithes (with enormously of vitellus - example: Cephalopod S, Sauropsides (Reptile S, Bird X))
• the ovocytes centrolécithes (with enormously of vitellus in the center - example: majority of the Arthropod S)

See too

• vitelline Gland