Extraembryonic membrane
The extraembryonic membranes are four membranes 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 zygote, but are not considered part of the embryo. They typically perform roles in nutrition, gas exchange and waste removal.{{cite book|author1=William K. Purves|author2=Gordon H. Orians|author3=H. Craig Heller|title=Life: The Science of Biology|date=2003|publisher=W. H. Freeman|isbn=978-0-7167-9856-9|page=[https://books.google.com/books?id=3_cIZqwSfdYC&pg=PA423 423]}}
In amniotes
There are four standard extraembryonic membranes in amniotes, i.e. reptiles (including birds) and mammals:
- the yolk sac which surrounds the yolk
- 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 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.{{cite book|author1=Noble S. Proctor|author2=Patrick J. Lynch|title=Manual of Ornithology: Avian Structure & Function|date=1993|publisher=Yale University Press|isbn=0-300-05746-6|page=[https://books.google.com/books?id=nQ3vLQkEWQMC&pg=PA234 234]}}
In humans and other mammals they are more usually called fetal membranes.
In insects
The extraembryonic membranes in insects include a serous membrane (serosa) originating from blastoderm cells, an amnion or amniotic cavity whose expression is controlled by the Zerknüllt gene, and a yolk sac. {{cite journal |last1=Schmitt-Ott |first1=Urs |last2=Kwan |first2=Chun Wai |title=Morphogenetic functions of extraembryonic membranes in insects |journal=Current Opinion in Insect Science |date=2016-02-11 |volume=13 |pages=86–92 |doi=10.1016/j.cois.2016.01.009 |pmid=27436557 |doi-access=free }}