Extra Embryonic Membranes in Birds and Mammals: Types, Development and Functions
Introduction
During embryonic development of birds, reptiles and mammals, specialized membranes develop outside the embryo to support growth, nutrition, respiration, excretion and protection.
What are Extra Embryonic Membranes?
Extra embryonic membranes are embryonic structures that do not form part of the adult body but assist the developing embryo by providing nutrition, gas exchange, protection and waste disposal.
Types of Extra Embryonic Membranes
Four extra embryonic membranes are found in terrestrial vertebrates:
- Amnion
- Yolk Sac
- Allantois
- Chorion
Amnion
The amnion surrounds the developing embryo and forms a fluid-filled amniotic sac.
Main Functions
- Protection from desiccation.
- Shock absorption.
- Prevents friction against shell.
- Allows movement of embryo.
Yolk Sac
The yolk sac surrounds the yolk and provides nutrients to the growing embryo.
Main Functions
- Digestion and absorption of yolk.
- Respiration.
- Blood cell formation.
- Haemopoiesis.
Allantois
The allantois develops as an outgrowth of the hindgut and serves excretory and respiratory functions.
Main Functions
- Stores nitrogenous waste.
- Participates in gas exchange.
- Forms chorioallantoic membrane.
Chorion (Serosa)
The chorion is the outermost membrane that encloses the embryo and other extra embryonic membranes.
It later combines with allantois to form the respiratory surface.
Development of Extra Embryonic Membranes
During neurulation, the lateral plate mesoderm splits into somatic and splanchnic layers, which subsequently form the extra embryonic membranes.
Development of Yolk Sac
- First extra embryonic membrane to appear.
- Develops from extra embryonic splanchnopleure.
- Surrounds the yolk completely.
- Connected to embryo by yolk stalk.
- Contains vitelline blood vessels.
Development of Amnion and Chorion
- Derived from extra embryonic somatopleure.
- Begin as cephalic amniotic fold.
- Lateral and caudal folds complete enclosure.
- Amnion forms inner sac.
- Chorion forms outer sac.
Development of Allantois
- Appears during the third day of incubation.
- Arises from hindgut endoderm.
- Expands rapidly between days 4–10.
- Fuses with chorion to form chorioallantoic membrane.
- Develops extensive blood supply.
Functions of Extra Embryonic Membranes
| Membrane | Main Function |
|---|---|
| Yolk Sac | Nutrition and blood formation |
| Amnion | Protection and fluid environment |
| Chorion | Respiration and nutrition |
| Allantois | Waste storage and respiration |
Frequently Asked Questions
What are extra embryonic membranes?
Special membranes that support the developing embryo.
How many extra embryonic membranes are present in birds?
Four: amnion, yolk sac, allantois and chorion.
Which membrane stores waste products?
Allantois.
Which membrane surrounds the embryo directly?
Amnion.
Which membrane provides nutrition?
Yolk sac.