Extraembryonic tissue

Extraembryonic tissue includes the four extraembryonic membranes which assist in the development of the animal embryo. The membranes occur in a range of animals from humans to insects. The tissue originates from the zygote, but is not considered part of the embryo. The membranes typically perform roles in nutrition, gas exchange and waste removal. ^[1]

In amniotes

There are four standard extraembryonic membranes in amniotes, i.e. reptiles (including birds) and mammals:

1. the yolk sac which surrounds the yolk
2. the amnion which surrounds and cushions the embryo
3. 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
4. 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. ^[2]

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. ^[3]

References

1. William K. Purves; Gordon H. Orians; H. Craig Heller (2003). Life: The Science of Biology. W. H. Freeman. p.  423. ISBN   978-0-7167-9856-9.
2. Noble S. Proctor; Patrick J. Lynch (1993). Manual of Ornithology: Avian Structure & Function. Yale University Press. p.  234. ISBN   0-300-05746-6.
3. Schmitt-Ott, Urs; Kwan, Chun Wai (2016-02-11). "Morphogenetic functions of extraembryonic membranes in insects". Current Opinion in Insect Science. 13: 86–92. doi: 10.1016/j.cois.2016.01.009 . PMID   27436557.