Extraembryonic Membranes: Amnion & Chorion

Quick answer: Extraembryonic membranes are supporting structures built from the embryo's own tissues that lie outside the embryo's body. In amniotes (reptiles, birds, mammals) four develop: the amnion (protective fluid sac), chorion (outer exchange membrane), allantois (waste storage and respiration) and yolk sac (nutrition and early blood formation).

Key Facts: Extraembryonic Membranes

• Extraembryonic membranes develop from the zygote but lie outside the embryo. They handle nutrition, gas exchange, protection, and waste disposal: the embryo's life-support system.
• There are four: amnion, chorion, allantois and yolk sac. You find them only in amniotes (reptiles, birds, mammals).
• The amnion wraps the embryo in protective amniotic fluid; the chorion is the outermost membrane and forms the fetal part of the placenta.
• The allantois stores nitrogenous waste (uric acid in birds) and helps with gas exchange; the yolk sac digests yolk and makes the first blood cells.
• Amnion + chorion develop from somatopleure (ectoderm + somatic mesoderm); yolk sac + allantois develop from splanchnopleure (endoderm + splanchnic mesoderm).
• Fishes and amphibians (anamniotes) never evolved these membranes, because their eggs develop in water.

Consider this: a hen lays her egg on dry straw, far from any pond. Inside that shell, a living embryo grows for three weeks. Why doesn't it dry out? Where does its waste go? How does it breathe? The answer is four brilliant membranes that the embryo builds around itself: the amnion, the chorion, the allantois and the yolk sac. They are supporting structures made from the embryo's own tissues, yet they never become part of the embryo's body. And together, they are the reason vertebrates could leave the water and conquer dry land.

Labeled diagram of an amniotic egg: the four extraembryonic membranes (amnion, chorion, allantois, yolk sac) around the embryo. Diagram: derived from bitmap by Horst Frank, SVG by -xfi-, CC BY-SA 3.0, via Wikimedia Commons.

Amniotes vs Anamniotes

Vertebrates split into two great camps based on whether their embryos build these membranes:

• Amniotes (have the membranes; reproduce on land or internally)
  • Reptiles: shelled eggs laid on land; all four membranes well developed
  • Birds: shelled eggs; allantois doubles as the respiratory organ (chorioallantoic membrane)
  • Mammals: no shelled egg, but all four membranes still form; the chorion builds the placenta
• Anamniotes (no membranes; eggs must develop in water or very moist places)
  • Fishes: eggs laid in water; wastes diffuse away, so no waste-storage membrane is needed
  • Amphibians: eggs laid in water or moist places; no amnion at all

The evolution of the amniotic egg was one of the great turning points in vertebrate history. It is what allowed full colonization of land.

The Four Membranes: Structure, Origin and Function

Now the heart of the topic. Study this table by asking three questions for each membrane. Where does it come from? What does it look like? What is its job?

Membrane	Germ-layer origin	Structure	Main functions
Amnion	Somatopleure (ectoderm + somatic mesoderm)	Innermost fold over the embryo; encloses the amniotic cavity filled with amniotic fluid	Protection: cushions against mechanical shock; prevents desiccation; allows free movement of the embryo
Chorion	Somatopleure (ectoderm + somatic mesoderm)	Outermost membrane surrounding embryo and all other membranes	Gas exchange with the environment; in mammals forms the fetal part of the placenta (chorionic villi)
Allantois	Splanchnopleure (endoderm + splanchnic mesoderm)	Sac-like outgrowth from the ventral hindgut	Waste storage: stores nitrogenous waste (uric acid in birds/reptiles); gas exchange via the chorioallantoic membrane; in mammals contributes to the umbilical cord and placenta
Yolk sac	Splanchnopleure (endoderm + splanchnic mesoderm)	Membrane enclosing the yolk mass	Nutrition: digests and absorbs yolk ("extraembryonic gut"); site of the first blood cells and blood vessels; vestigial in humans

Membrane by Membrane: A Hierarchical View

For deeper answers, expand each membrane into its origin, structure, functions and fate:

• Amnion (the protector)
  • Origin: inner fold of the somatopleure (ectoderm + somatic mesoderm)
  • Structure: thin membrane enclosing the amniotic cavity, filled with amniotic fluid secreted by the amnion and embryo
  • Functions: cushions mechanical shocks; prevents desiccation; keeps temperature even; allows free embryonic movement
  • Fate: in humans forms the amniotic sac; "breaking of the waters" at childbirth is its rupture
• Chorion (the outer exchange surface)
  • Origin: outer fold of the somatopleure (ectoderm + somatic mesoderm)
  • Structure: outermost membrane enclosing the embryo and all other membranes
  • Functions: gas exchange with the environment; protective barrier
  • Fate: in mammals develops chorionic villi that invade the uterine wall: the fetal portion of the placenta
• Allantois (the waste manager and extra lung)
  • Origin: splanchnopleure (endoderm + splanchnic mesoderm); buds from the ventral hindgut
  • Structure: sac-like outgrowth that expands between amnion and chorion
  • Functions: stores nitrogenous waste (insoluble uric acid in birds/reptiles), the "extraembryonic kidney"; fuses with the chorion to form the chorioallantoic membrane, the embryo's "lung" pressed against the shell
  • Fate: in mammals small and vestigial; its blood vessels become the umbilical arteries and vein
• Yolk sac (the food store and first blood factory)
  • Origin: splanchnopleure (endoderm + splanchnic mesoderm)
  • Structure: membrane wrapping the yolk mass, connected to the embryo's gut
  • Functions: digests and absorbs yolk ("extraembryonic gut"); site of the first blood cells and blood vessels; origin of primordial germ cells
  • Fate: tiny and vestigial in humans (no yolk in the human egg), yet still the first blood factory

Extraembryonic tissues during amniote development: how the membranes are arranged around the embryo. Image: Guojun Sheng and Ann C Foley, CC BY 2.5, via Wikimedia Commons.

How They Form (Chick Embryo)

Reading the table is one thing; watching the membranes form is another. In the chick embryo, it happens in three steps:

1. Amnion and chorion folds: folds of somatopleure rise around the embryo and meet above it, creating the inner amnion and the outer chorion.
2. Yolk sac: the yolk sac grows around the yolk as a splanchnopleuric envelope, digesting yolk and passing nutrients to the embryo.
3. Allantois: the allantois buds off from the hindgut, expands between amnion and chorion, stores uric acid, and fuses with the chorion to form the chorioallantoic membrane: the embryo's "lung," pressed against the shell for gas exchange.

Together, these four extraembryonic membranes turn the cleidoic egg into a self-contained life-support system for the developing chick. This sequence follows cleavage and gastrulation, the two earlier chapters of embryonic development.

Extraembryonic Membranes in Humans

"But humans don't lay eggs." True, and yet we build all four membranes. Here is what each one does in us:

• Amnion: forms the amniotic sac. "Breaking of the waters" at childbirth is simply the rupture of the amnion.
• Chorion: develops chorionic villi that invade the uterine wall: the fetal portion of the placenta.
• Allantois: small and vestigial, but its blood vessels become the umbilical arteries and vein.
• Yolk sac: tiny and vestigial (there is no yolk in the human egg), yet still the first site of blood cell formation and the origin of primordial germ cells. If it has no yolk, why bother making one? Because evolution rarely throws away a working blood factory.

Examiner tip

When this topic comes as a short question, present the four membranes as a table with three columns: membrane, origin, function. Examiners check two pairings most: amnion goes with protection and amniotic fluid, and allantois goes with waste storage plus gas exchange. Now the classic trap: the amnion and chorion both come from somatopleure, while the yolk sac and allantois both come from splanchnopleure. Memorize it as "AC = somatopleure, YA = splanchnopleure". And one line to close with: extraembryonic membranes are found only in amniotes.

Frequently Asked Questions

What are the four extraembryonic membranes?

The amnion (protective fluid sac), chorion (outermost exchange membrane), allantois (waste storage and respiration) and yolk sac (nutrition and early blood formation).

Which extraembryonic membrane forms the placenta?

The chorion: its chorionic villi grow into the uterine wall and form the fetal part of the placenta (aided by the allantois in most mammals).

What is the function of amniotic fluid?

Amniotic fluid, secreted into the amniotic cavity by the amnion (and embryo), cushions the embryo against mechanical shocks, prevents desiccation and sticking of tissues, maintains even temperature, and allows free movement for musculoskeletal development.

Why don't fishes and amphibians have extraembryonic membranes?

They are anamniotes: their eggs develop in water, so there is no danger of desiccation and wastes diffuse away easily. The membranes evolved specifically for reproduction on dry land.

Which membrane is called the extraembryonic kidney?

The allantois: in reptiles and birds it stores the embryo's nitrogenous waste (mainly insoluble uric acid), functioning like a kidney outside the embryo.

Do the extraembryonic membranes become part of the embryo's body?

No. They develop from the zygote's tissues but lie outside the embryo and never become part of its body: they are temporary life-support structures, discarded at hatching or birth (except where the chorion and allantois contribute vessels to the placenta and umbilical cord).

Practice MCQs

Q1. The extraembryonic membrane that encloses the embryo in a fluid-filled sac is the:

1. Chorion
2. Amnion
3. Allantois
4. Yolk sac

Answer: B — The amnion forms the amniotic cavity containing the protective amniotic fluid.

Q2. The allantois develops as an outgrowth of the:

1. Foregut
2. Midgut
3. Hindgut
4. Neural tube

Answer: C — The allantois is a splanchnopleuric evagination from the ventral hindgut.

Q3. Which pair of membranes develops from the somatopleure?

1. Yolk sac and allantois
2. Amnion and chorion
3. Amnion and allantois
4. Chorion and yolk sac

Answer: B — Amnion and chorion are somatopleuric (ectoderm + somatic mesoderm); yolk sac and allantois are splanchnopleuric.

Q4. The first blood cells of the embryo are formed in the:

1. Amnion
2. Chorion
3. Yolk sac
4. Allantois

Answer: C — The yolk sac is the primary site of early hematopoiesis (blood cell formation).

Q5. Extraembryonic membranes are absent in:

1. Birds
2. Reptiles
3. Amphibians
4. Mammals

Answer: C — Amphibians (and fishes) are anamniotes; their eggs develop in water without these membranes.

Q6. In mammals, the fetal part of the placenta is formed by the:

1. Chorion
2. Amnion
3. Yolk sac
4. Allantois

Answer: A — The chorion is the outermost membrane and develops chorionic villi that invade the uterine wall, forming the fetal portion of the placenta.

Q7. "Breaking of the waters" at childbirth is the rupture of the:

1. Chorion
2. Yolk sac
3. Allantois
4. Amnion

Answer: D — The amnion forms the amniotic sac; its rupture at childbirth is what is called the "breaking of the waters".

Q8. The primordial germ cells of the human embryo originate in the:

1. Yolk sac
2. Amnion
3. Chorion
4. Allantois

Answer: A — Although tiny and vestigial in humans, the yolk sac is still the first site of blood cell formation and the origin of primordial germ cells.

Q9. The chorioallantoic membrane of the chick embryo:

1. Digests and absorbs the yolk
2. Cushions the embryo against mechanical shock
3. Becomes the umbilical vein
4. Acts as the embryo's "lung", pressed against the shell for gas exchange

Answer: D — The allantois expands between amnion and chorion and fuses with the chorion to form the chorioallantoic membrane, the embryo's "lung" for gas exchange.

Q10. In the human embryo, the allantois:

1. Stores large amounts of uric acid
2. Is small and vestigial — its blood vessels become the umbilical arteries and vein
3. Forms the amniotic sac
4. Digests the yolk mass

Answer: B — The human allantois is small and vestigial; its blood vessels become the umbilical arteries and vein, and it contributes to the umbilical cord.

Q11. The outermost extraembryonic membrane is the:

1. Amnion
2. Chorion
3. Allantois
4. Yolk sac

Answer: B — The chorion is the outermost membrane, surrounding the embryo and all the other membranes.

Q12. In birds and reptiles, the allantois stores nitrogenous waste mainly as:

1. Ammonia
2. Urea
3. Uric acid
4. Creatinine

Answer: C — The allantois stores the embryo's nitrogenous waste as insoluble uric acid, which is why it is called the extraembryonic kidney.

Short Questions

Q1. What are extraembryonic membranes, and which four are found in amniotes? See answer

Extraembryonic membranes are supporting structures that arise from the embryo's own tissues (ultimately the zygote) but lie outside the embryo's body. They never become part of the body; they are temporary life-support organs handling nutrition, gas exchange, protection and waste disposal during development.

In amniotes (reptiles, birds, mammals) four develop: the amnion (innermost, protective fluid sac), the chorion (outermost, exchange membrane), the allantois (sac from the hindgut storing waste and aiding respiration) and the yolk sac (nutrition and first blood formation).

They are found only in amniotes: fishes and amphibians (anamniotes) never evolved them, because their eggs develop in water where desiccation and waste accumulation are not problems. The membranes were the key innovation that let vertebrates reproduce on dry land.

Q2. Distinguish between the amnion and the chorion in structure and function. See answer

The amnion is the innermost membrane: an inner fold of the somatopleure (ectoderm + somatic mesoderm) that encloses the amniotic cavity filled with amniotic fluid. Its functions are all protective: it cushions mechanical shocks, prevents desiccation, keeps temperature even and allows free embryonic movement.

The chorion is the outermost membrane: the outer fold of the same somatopleure, surrounding the embryo and all other membranes. Its main role is gas exchange with the environment, plus general protection.

In mammals the chorion takes on a spectacular extra job: it develops chorionic villi that invade the uterine wall and form the fetal portion of the placenta. The amnion, meanwhile, becomes the amniotic sac whose rupture at childbirth is the "breaking of the waters". Both are somatopleuric: memorize "AC = somatopleure".

Q3. Why is the allantois called the "extraembryonic kidney"? See answer

The allantois is a sac-like outgrowth of the ventral hindgut, formed from splanchnopleure (endoderm + splanchnic mesoderm). In reptiles and birds it expands between the amnion and chorion and collects the embryo's nitrogenous waste, mainly insoluble uric acid: exactly the job a kidney does, but performed outside the embryo's body, which is why it is called the extraembryonic kidney.

It has a second job too: fusing with the chorion to form the chorioallantoic membrane, the embryo's "lung" pressed against the eggshell for gas exchange. So one membrane handles both waste and respiration.

In mammals the allantois is small and vestigial, but not useless: its blood vessels become the umbilical arteries and vein, and it contributes to the umbilical cord and placenta. The shift from waste sac to vascular stalk is a textbook example of evolutionary repurposing.

Q4. Which germ-layer combinations form the four extraembryonic membranes? See answer

The four membranes are built from two compound layers. Somatopleure (ectoderm + somatic mesoderm) forms the amnion and the chorion; splanchnopleure (endoderm + splanchnic mesoderm) forms the yolk sac and the allantois.

Examiners love to scramble these, so memorize the pairing as "AC = somatopleure, YA = splanchnopleure": Amnion and Chorion from somatopleure, Yolk sac and Allantois from splanchnopleure. Note the logic: the outer protective pair is ectoderm-based, while the inner nutritive/waste pair is endoderm-based.

This pairing also mirrors function: the somatopleuric membranes (amnion, chorion) face outward and handle protection and exchange, while the splanchnopleuric membranes (yolk sac, allantois) face inward toward the gut and yolk and handle nutrition and waste.

Q5. Why do fishes and amphibians lack extraembryonic membranes? See answer

Fishes and amphibians are anamniotes: they never evolved these membranes because their eggs develop in water. In water there is no danger of desiccation, so a protective fluid sac is unnecessary, and nitrogenous wastes diffuse away into the surrounding water, so a waste-storage sac is unnecessary too.

By contrast, on land an egg faces two lethal problems: drying out and poisoning itself with its own waste. The amniotic egg solved both: the amnion provides a private aquatic environment and the allantois a waste dump, while the chorion and chorioallantoic membrane breathe through the shell.

That is why the evolution of extraembryonic membranes counts as one of the great turning points in vertebrate history: it freed reproduction from water and allowed the full colonization of land by reptiles, birds and mammals.

Q6. What is the fate of each extraembryonic membrane in the human embryo? See answer

The amnion forms the amniotic sac that holds the fetus in amniotic fluid; its rupture at the start of labour is the "breaking of the waters". The chorion develops chorionic villi that invade the uterine wall and form the fetal portion of the placenta, the organ of exchange between mother and fetus.

The allantois is small and vestigial in humans, but its blood vessels persist as the umbilical arteries and vein, and it contributes to the umbilical cord. The yolk sac is also tiny and vestigial (the human egg has no yolk), yet it remains the first site of blood cell formation and the origin of primordial germ cells.

The human case teaches an evolutionary lesson: even when the original function (yolk digestion, waste storage) disappears, the membranes are repurposed for blood formation, germ-cell origin and placental exchange, because development rarely discards a working structure.

References

1. Hickman, C.P., Roberts, L.S. & Larson, A. Integrated Principles of Zoology, 18th ed. McGraw-Hill. (Extraembryonic membranes; amniote egg.)
2. Miller, S.A. & Harley, J.P. Zoology, McGraw-Hill. (Fetal membranes; placentation.)
3. OpenStax, Biology 2e, Chapter 43: Animal Reproduction and Development: https://openstax.org/details/books/biology-2e.
4. NCBI Bookshelf — Developmental Biology: Extraembryonic membranes: https://www.ncbi.nlm.nih.gov/books/.

Read next

• Gastrulation in Frog: Stages & Blastopore
• Cleavage in Eggs: Holoblastic vs Meroblastic
• Monotremes vs Marsupials vs Placentals: Key Differences