Extra Embryonic Membranes:

Definition of Extra Embryonic Membrane: The blastoderm in birds , reptiles and mammals not only gives rise to embryo, but also to certain structures that lie outside the embryo. These extra embryonic structures are called Extra Embryonic Membrane.

Types of extra embryonic membranes Yolk sac Amnion Chorion Allantois

Yolk Sac Origin: From the extra embryonic splanchnopleur (endoderm on the inner and Splanchnic mesoderm on the outer side). Yolk sac has a wide opening into the midgut . As development proceeds the passages of midgut is reduced to a narrow umblical stock.. The opening of umblical stock into the midgut is called umblicus. Yolk sac spreads more and more around the yolk. Passage left on ventral side to absorb albumen. On the first day mesoderm joins it and on the ninth day it is fully formed.

Functions of the Yolk Sac : Digest the yolk . Transfer the products of digestion to the embryo. Digestive surface increased by force off the walls of the yolk sacs called yolk sac septa . In Mammals yolk sac is less nutritive organ then Reptiles and Aves . In Mammals it functions as embryonic haemopoietic organ (site for blood cells formation).

After Hatching : Yolk is digested. Yolk sac become small. It is withdrawn into the intestine and the umbilicus closes.

Amnion Origin: From somatopleur (ectoderm + somatic mesoderm). It develops as folds of somatopleur. Fold over the head of the embryo is called head of the embryo. Fold over the tail is called tail of the embryo. The folds converge above the embryo, meet and fuse. The points of fusion is called seroamniotic connection. Fusion results two membrane over the embryo: inner membrane = amnion and outer membrane is called Chorion.

Functions of Amnion : Amnion protects the embryo from shock and injury. Amniotic fluid prevents its desiccation.

Chorion From somatopleur(ectoderm + somatic mesoderm). Origin: From somatopleur(ectoderm + somatic mesoderm). Fusion of head fold of amnion and tail fold of amnion produces two membranes over the embryo. Inner layer is called amnion and outer is called chorion. Chorion is also called false amnion. .

Functions of chorion : It protects the foetus. Provides place for the growth of allantois. Helps in the formation of the placenta.

Allantois Origin: From Splanchnopleur(endoderm on inner side and Splanchnic mesoderm on the outer side). It develops from the floor of hindgut of the foetus. It goes into choriotic cavity. Splanchnic mesoderm off Allantois fuses with somatic mesoderm of the chorion forming Allantochorion. Allantochorion develops allantoic artries and veins . Allantois stock (connection between allantios and hind gut). Umbilical chord (somatopleur surround the allantois stock and umbilical stock). At hatching time umblical chord breaks . Allantois left behind. Place of attachment of the umblical chord to the body heals up. Permanent scarp, the umblicus is formed.

Functions of Allantois : Store insoluble nitrogenous waste matter, uric acid. Functions as extra embryonic lung . Gaseous exchange taking place between blood and external air through it. Carries on excretion ,respiration and nutrition . Allantois function as a soft, elastic cushion for protecting the embryo from shock . Allantois helps in a formation of umbilical chord.

Amniotic cavity Definition: The cavity enclosed between the embryo and amnion is called amniotic cavity. The fluid filled in this cavity is called amniotic cavity. The embryo floats in this fluid . functions of amniotic cavity : Serves as water cushion to protect the embryo. Prevents the desiccation of the embryo. Check the stagnation of blood in the embryonic blood vessels .

Choriotic cavity Definition: Functions of choriotic cavity The space between the amnion and the chorion is called choriotic cavity. Functions of choriotic cavity Fluid of chorionic cavity provides the protection to the embryo. Provides the space for the growth of the allantios . Plays an important role in gaseous exchange through the porous shell.

How extra Embryonic Membranes helps in the Placental Formation ? Allantochorion comes in contact with uterine wall. Develops large number of projections called secondary or chorionic villi. They are formed on tenth day of development. Villi penetrates in depressions called crypts. Crypts become vascular.

Intimate connection established between foetal membranes and uterine wall is called placenta. The part of placenta contributed by foetus (Allantochorion) is called foetal placenta. Part shared by mother (uterine wall) called maternal placenta.

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