Extra Embryonic Membranes in Birds and Mammals: Types, Development and Functions
Complete Guide • Embryology • Amnion • Chorion • Yolk Sac • Allantois • NEET & University Notes
Key Takeaways
- Understand the concept of extra embryonic membranes.
- Learn the structure and functions of the four embryonic membranes.
- Study the development of amnion, chorion, yolk sac and allantois.
- Understand the role of these membranes in nutrition, respiration, excretion and protection.
- Important for Class 11 Biology, NEET, CUET and university examinations.
|
| Extra Embryonic Membranes in Chick Embryo |
Introduction
During the embryonic development of birds, reptiles and mammals, several specialized membranes develop outside the embryo. These structures are known as extra embryonic membranes. Although they do not become part of the body of the adult organism, they play essential roles in protecting and supporting the developing embryo throughout embryogenesis.
These membranes provide nutrition, respiration, excretion, protection against mechanical injury and help maintain a suitable environment for normal embryonic development. In oviparous vertebrates such as birds, they are especially important because the embryo develops inside the egg without direct maternal support.
Quick Fact
The four extra embryonic membranes found in amniotes are Amnion, Chorion, Yolk Sac and Allantois. Together they ensure proper growth, nourishment and survival of the developing embryo.
What are Extra Embryonic Membranes?
Extra embryonic membranes are temporary embryonic structures that develop outside the embryo but remain closely associated with it during development. They assist in nutrition, respiration, excretion, protection and circulation until the embryo becomes fully developed.
Unlike embryonic tissues, these membranes do not contribute directly to the organs of the adult organism. Instead, they provide a supportive environment that enables successful embryonic growth both inside eggs and within the uterus of mammals.
Definition
Extra embryonic membranes are specialized membranes formed during embryonic development that support the embryo by providing nourishment, protection, respiration and waste removal without forming part of the adult body.
Types of Extra Embryonic Membranes
Four major extra embryonic membranes develop in birds, reptiles and mammals. Each membrane performs a specific function essential for embryonic survival.
🥚 Amnion
Forms the fluid-filled amniotic sac that protects the embryo.
🟡 Yolk Sac
Provides nutrition, blood formation and early circulation.
🫁 Allantois
Functions in respiration, excretion and waste storage.
🫧 Chorion
Outermost membrane responsible for gas exchange and placental formation in mammals.
Important Note
The fusion of the chorion and allantois forms the chorioallantoic membrane, which serves as the major respiratory surface in bird embryos and contributes to placental development in mammals.
Amnion
The amnion is the innermost extra embryonic membrane that directly surrounds the developing embryo. It encloses the embryo within a fluid-filled cavity called the amniotic cavity, which contains the amniotic fluid. This membrane provides a safe and stable environment throughout embryonic development.
In birds, reptiles and mammals, the amnion protects the embryo from mechanical shocks, prevents dehydration and allows free movement required for proper growth and development.
Functions of Amnion
- Protects the embryo from mechanical injury.
- Prevents desiccation (drying).
- Acts as a shock absorber.
- Allows free movement of the embryo.
- Maintains a constant temperature around the embryo.
- Prevents adhesion between the embryo and surrounding membranes.
Yolk Sac
The yolk sac is the first extra embryonic membrane to develop during embryogenesis. It surrounds the yolk and serves as the primary source of nourishment for the growing embryo, especially in birds and reptiles.
Besides nutrition, the yolk sac is also involved in early blood formation and the development of the embryo's circulatory system.
Functions of Yolk Sac
- Stores and supplies nutrients to the embryo.
- Digests and absorbs yolk.
- Produces the first blood cells.
- Forms early blood vessels (vitelline circulation).
- Participates in haemopoiesis.
Allantois
The allantois develops as an outgrowth from the hindgut of the embryo. It enlarges rapidly during development and performs important functions related to excretion, respiration and circulation.
In birds, the allantois stores nitrogenous wastes and later fuses with the chorion to form the highly vascular chorioallantoic membrane, which acts as the respiratory organ of the embryo.
Functions of Allantois
- Stores nitrogenous waste products.
- Participates in gaseous exchange.
- Forms the chorioallantoic membrane.
- Provides a rich blood supply for respiration.
- Assists in water balance.
Chorion (Serosa)
The chorion, also known as the serosa, is the outermost extra embryonic membrane surrounding all the other embryonic membranes. It encloses the embryo together with the amnion, yolk sac and allantois.
In birds, the chorion combines with the allantois to form the chorioallantoic membrane, which functions as the principal respiratory surface. In mammals, it contributes to the formation of the placenta.
Functions of Chorion
- Protects the embryo and surrounding membranes.
- Participates in gaseous exchange.
- Forms the chorioallantoic membrane with the allantois.
- Contributes to placental formation in mammals.
- Supports exchange of nutrients and wastes.
Comparison of the Four Extra Embryonic Membranes
| Membrane | Main Function | Important Feature |
|---|---|---|
| Amnion | Protection | Contains amniotic fluid |
| Yolk Sac | Nutrition | First membrane to develop |
| Allantois | Respiration & Excretion | Stores nitrogenous wastes |
| Chorion | Gas Exchange | Outermost membrane |
Key Characteristics
- ✔ Four extra embryonic membranes are present in amniotes.
- ✔ Amnion directly surrounds the embryo.
- ✔ Yolk sac provides nutrition and forms early blood cells.
- ✔ Allantois stores waste and participates in respiration.
- ✔ Chorion is the outermost membrane.
- ✔ Chorion and allantois together form the chorioallantoic membrane.
Exam Tips
- 📝 Amnion → Protection.
- 📝 Yolk Sac → Nutrition.
- 📝 Allantois → Respiration + Excretion.
- 📝 Chorion → Gas exchange + Placenta.
- 📝 Chorioallantoic membrane is a favorite NEET question.
Development of Extra Embryonic Membranes
Extra embryonic membranes begin to develop during the early stages of embryogenesis. As gastrulation and neurulation proceed, the lateral plate mesoderm splits into two layers—the somatic mesoderm (somatopleure) and the splanchnic mesoderm (splanchnopleure). Together with the ectoderm and endoderm, these layers give rise to the four extra embryonic membranes.
Each membrane develops from a specific embryonic tissue and performs specialized functions essential for the survival, protection and growth of the embryo.
Origin of Extra Embryonic Membranes
- Amnion → Somatopleure
- Chorion → Somatopleure
- Yolk Sac → Splanchnopleure
- Allantois → Endoderm of Hindgut + Splanchnic Mesoderm
Development of Yolk Sac
The yolk sac is the first extra embryonic membrane to develop. It originates from the extra embryonic splanchnopleure and gradually surrounds the yolk, forming an important nutritive organ for the embryo.
Developmental Stages
- First extra embryonic membrane to appear.
- Develops from extra embryonic splanchnopleure.
- Encloses the yolk completely.
- Connected to the embryo through the yolk stalk.
- Vitelline blood vessels develop within its wall.
- Supports early haemopoiesis and nutrient absorption.
Development of Amnion and Chorion
The amnion and chorion develop simultaneously from the extra embryonic somatopleure. During development, folds arise around the embryo and gradually fuse over it to form the protective amniotic sac and the outer chorionic membrane.
| Stage | Development |
|---|---|
| 1 | Cephalic amniotic fold appears. |
| 2 | Lateral amniotic folds develop. |
| 3 | Caudal fold completes enclosure. |
| 4 | Inner membrane forms the Amnion. |
| 5 | Outer membrane forms the Chorion. |
Important Point
The space enclosed by the amnion becomes filled with amniotic fluid, which cushions and protects the embryo throughout development.
Development of Allantois
The allantois develops as a diverticulum from the hindgut endoderm. During incubation it enlarges rapidly and becomes highly vascular. Eventually it fuses with the chorion to form the chorioallantoic membrane, which functions as the respiratory organ of the developing chick embryo.
Developmental Stages
- Appears around the third day of incubation.
- Arises from the hindgut endoderm.
- Rapid enlargement between days 4–10.
- Develops an extensive blood vessel network.
- Fuses with the chorion to form the chorioallantoic membrane.
- Stores nitrogenous waste products.
Development Flow Chart
Fertilization
⬇
Cleavage
⬇
Blastulation
⬇
Gastrulation
⬇
Neurulation
⬇
Formation of Somatopleure & Splanchnopleure
⬇
Development of Extra Embryonic Membranes
⬇
Amnion • Chorion • Yolk Sac • Allantois
Important Notes
- ✔ Yolk sac is the first extra embryonic membrane to develop.
- ✔ Amnion and chorion arise from the somatopleure.
- ✔ Yolk sac develops from the splanchnopleure.
- ✔ Allantois originates from the hindgut.
- ✔ Chorioallantoic membrane acts as the respiratory organ in bird embryos.
Memory Tricks
- 🥚 Amnion → Around the embryo.
- 🟡 Yolk Sac → Yolk = Nutrition.
- 🫁 Allantois → Air + Waste.
- 🌐 Chorion → Outer covering.
- 📝 Remember: AYAC → Amnion, Yolk Sac, Allantois, Chorion.
Functions of Extra Embryonic Membranes
Each extra embryonic membrane performs a unique function during embryonic development. Together, these membranes provide nutrition, protection, respiration, circulation and waste removal, ensuring proper growth and survival of the developing embryo.
| Membrane | Main Function | Additional Role |
|---|---|---|
| Amnion | Protection of embryo | Provides fluid-filled environment |
| Yolk Sac | Nutrition | Blood cell formation and circulation |
| Allantois | Respiration and Excretion | Stores nitrogenous wastes |
| Chorion | Gas exchange | Forms placenta in mammals |
Detailed Functions of Each Membrane
Amnion
- Protects the embryo from mechanical shocks.
- Prevents dehydration.
- Maintains a constant fluid environment.
- Allows free movement of the embryo.
- Prevents adhesion between embryo and surrounding tissues.
Yolk Sac
- Supplies nutrients from the yolk.
- Produces the first blood cells.
- Forms vitelline blood vessels.
- Participates in early haemopoiesis.
- Supports embryonic circulation.
Allantois
- Stores nitrogenous wastes.
- Participates in gaseous exchange.
- Forms the chorioallantoic membrane.
- Develops a rich vascular network.
- Helps regulate water balance.
Chorion
- Forms the outer protective covering.
- Participates in respiration.
- Combines with allantois to form the respiratory membrane.
- Forms fetal part of placenta in mammals.
- Supports exchange of nutrients and gases.
Comparison of the Four Extra Embryonic Membranes
| Feature | Amnion | Yolk Sac | Allantois | Chorion |
|---|---|---|---|---|
| Position | Innermost | Around yolk | Between yolk sac and chorion | Outermost |
| Main Function | Protection | Nutrition | Excretion & Respiration | Respiration |
| Contains | Amniotic Fluid | Yolk | Nitrogenous Waste | Embryonic Membranes |
| Importance | Shock absorber | Food supply | Waste storage | Gas exchange |
Clinical Significance
Extra embryonic membranes are of great importance in embryology, obstetrics and prenatal medicine. Abnormal development of these membranes may lead to congenital abnormalities, pregnancy complications or impaired fetal development.
- Amniotic fluid protects the fetus during pregnancy.
- Placenta develops mainly from the chorion.
- Abnormal amniotic fluid volume may indicate fetal disorders.
- Umbilical cord is connected with extra embryonic structures.
- Chorioallantoic membrane serves as an important respiratory organ in bird embryos.
Quick Revision
- ✔ Four extra embryonic membranes are present.
- ✔ Amnion surrounds the embryo directly.
- ✔ Yolk sac provides nutrition.
- ✔ Allantois stores nitrogenous waste.
- ✔ Chorion is the outermost membrane.
- ✔ Chorion + Allantois = Chorioallantoic membrane.
- ✔ Placenta develops mainly from the chorion in mammals.
Competitive Examination Points
- Frequently asked in NEET Biology.
- Important for Class 11 Biology and CUET.
- Remember the order: Amnion → Yolk Sac → Allantois → Chorion.
- Yolk sac is the first extra embryonic membrane to develop.
- Chorioallantoic membrane is a common MCQ topic.
Common Mistakes Students Make
- ❌ Confusing the chorion with the amnion.
- ❌ Assuming the yolk sac surrounds the embryo.
- ❌ Forgetting that the allantois stores nitrogenous wastes.
- ❌ Mixing up the origins of somatopleure and splanchnopleure.
- ❌ Forgetting that the chorion contributes to placental formation in mammals.
Frequently Asked Questions (FAQs)
1. What are extra embryonic membranes?
Extra embryonic membranes are temporary embryonic structures that develop outside the embryo and provide protection, nutrition, respiration and waste removal during embryonic development. These membranes do not become part of the adult body.
2. How many extra embryonic membranes are present in birds and mammals?
There are four extra embryonic membranes:
- 🥚 Amnion
- 🟡 Yolk Sac
- 🫁 Allantois
- 🌐 Chorion
3. Which extra embryonic membrane surrounds the embryo?
The amnion directly surrounds the embryo and forms the fluid-filled amniotic sac that protects the developing embryo from mechanical injury and dehydration.
4. Which membrane provides nutrition to the embryo?
The yolk sac surrounds the yolk and supplies nutrients to the developing embryo. It also produces the first blood cells and vitelline blood vessels.
5. What is the function of the allantois?
The allantois stores nitrogenous waste products, participates in gaseous exchange and later combines with the chorion to form the chorioallantoic membrane.
6. What is the function of the chorion?
The chorion is the outermost membrane that protects the embryo and participates in respiration. In mammals, it also contributes to the formation of the placenta.
7. Which membrane forms the placenta in mammals?
The chorion, together with maternal tissues, forms the fetal part of the placenta in mammals.
8. Which extra embryonic membrane develops first?
The yolk sac is the first extra embryonic membrane to develop during embryogenesis.
Conclusion
Extra embryonic membranes are essential adaptations that enable the successful development of embryos in birds, reptiles and mammals. These specialized membranes provide protection, nourishment, respiration, excretion and circulation throughout embryonic life. Although temporary in nature, they play a critical role in ensuring normal embryonic growth and survival.
A thorough understanding of the structure, development and functions of the amnion, yolk sac, allantois and chorion is fundamental for students preparing for Class 11 Biology, NEET, CUET, Nursing Entrance, B.Sc. Zoology and other competitive examinations.
Final Revision
- ✔ Four extra embryonic membranes are present in amniotes.
- ✔ Amnion protects the embryo with amniotic fluid.
- ✔ Yolk sac provides nutrition and forms early blood cells.
- ✔ Allantois stores nitrogenous wastes and helps in respiration.
- ✔ Chorion is the outermost membrane and forms the placenta in mammals.
- ✔ Chorion and allantois together form the chorioallantoic membrane.
- ✔ These membranes are temporary but essential for embryonic development.
More Topics
References
- NCERT Biology Class XI
- NCERT Biology Class XII
- Gilbert – Developmental Biology
- Langman's Medical Embryology
- Balinsky's Introduction to Embryology
- Campbell Biology
- Modern Embryology Textbooks