What does amnion mean?
The amnion is the delicate, transparent membrane that encloses an embryo or fetus in reptiles, birds, and mammals, creating the fluid-filled cavity known as the amniotic sac. Suspended within the protective amniotic fluid, the developing young is shielded from physical shock and kept moist, allowing development to proceed safely even on dry land — a key evolutionary advance for egg-laying vertebrates as well as mammals. In humans, the term belongs chiefly to medical and embryological language: doctors speak of the amnion rupturing during labour, and procedures such as amniocentesis draw fluid from the sac it forms. In casual usage, people more often say 'amniotic sac' than 'amnion'. Derived from the Greek word for a vessel used to catch the blood of slaughtered lambs, the name reflects ancient observation of birth membranes in livestock. Understanding the amnion is fundamental to grasping how vertebrates reproduce successfully away from water.
nounA thin membrane that encloses the embryo or fetus of reptiles, birds, and mammals, forming a fluid-filled sac in which the developing young is suspended and protected.
- The thin, fluid-filled membrane enclosing the embryo or fetus of reptiles, birds, and mammals; also called the amniotic sac.
"The developing foetus floats within the amnion, cushioned by the surrounding amniotic fluid."
"Ultrasound imaging showed the baby moving freely within the amnion."
"In birds and reptiles, the amnion allows the embryo to develop on land without drying out."
Also rendered as 'amnia' following Latin-style pluralisation, but 'amnions' is standard in modern medical writing.
"Comparative studies examined the structure of the amnions across different species of reptiles."
The word 'amnion' comes from the Greek for a lamb's trough — because ancient shepherds noticed newborn lambs arrived wrapped in this glistening membrane.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of amnion
The word 'amnion' derives from the ancient Greek ἄμνιον (amnion), a diminutive form related to ἀμνός (amnos), meaning 'lamb'; it referred to a bowl or trough used to catch the blood of sacrificial lambs, and was applied by Greek physicians to the fetal membrane, perhaps from its resemblance to such a vessel or its association with animal birth. It passed into New Latin and entered English in the early seventeenth century. Related English derivatives include 'amniotic' and 'amniocentesis'.
Related word forms
How amnion is actually used
Primarily used in medical, embryological, and veterinary contexts. In everyday speech the phrase 'amniotic sac' is more common than 'amnion'. The plural is usually 'amnions', though 'amnia' is an accepted Latinate plural.
Easily confused with amnion
The albumen is the white of an egg, whereas the amnion is the innermost membrane enclosing the embryo itself.