The extraembryonic membranes are four

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

s which assist in the development of an animal's embryo. Such membranes occur in a range of animals from humans to insects. They originate from the embryo, but are not considered part of it. They typically perform roles in nutrition, gas exchange, and waste removal. There are four standard extraembryonic membranes in birds, reptiles, and mammals: the

yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is ...

which surrounds the

yolk Among animals which produce eggs, 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 types of egg contain no yolk, for example ...

, the amnion which surrounds and cushions the embryo, the allantois which among avians stores embryonic waste and assists with the exchange of carbon dioxide with oxygen as well as the resorption of calcium from the shell, and the

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitell ...

which surrounds all of these and in avians successively merges with the allantois in the later stages of egg development to form a combined respiratory and excretory organ called the chorioallantois. The extraembryonic membranes in insects include a

serous membrane The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane ...

originating from

blastoderm A blastoderm ( germinal disc, blastodisc) is a single layer of embryonic epithelial tissue that makes up the blastula. It encloses the fluid filled blastocoel. Gastrulation follows blastoderm formation, where the tips of the blastoderm begins the ...

cells, an amnion or amniotic cavity whose expression is controlled by the

Zerknüllt ''Zerknüllt'' (zen, German for "crumpled") is a gene in the Antennapedia complex of ''Drosophila'' (fruit flies) and other insects, where it operates very differently from the canonical Hox genes in the same gene cluster. Comparison of Hox gen ...

gene, and a yolk sac. In humans and other mammals they are more usually called

fetal membranes The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals.. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic me ...

References

Embryology {{Animal-anatomy-stub