4 foetal membranes develop outside the embryo body to support it during pregnancy:

1. Yolk Sac — first membrane; nutrition early on; early haematopoiesis; primordial germ cells are specified from epiblast-derived progenitors and temporarily localize near the yolk sac before migrating. Regresses after placenta takes over.
2. Amnion — innermost; encloses the embryo in amniotic fluid; protects from shock + dehydration.
3. Allantois — outpouching of hindgut; in mammals expands to fuse with chorion to form chorioallantoic placenta. Stores fetal waste.
4. Chorion — outermost; in contact with maternal endometrium; site of nutrient/gas exchange = the embryo's part of the placenta.

Aves (Bird): All 4 membranes used — yolk sac (nutrition from yolk), allantois (gas exchange + waste storage), amnion (protection), chorion (gas exchange with shell).

FOETAL MEMBRANES — The Embryo's Life-Support System

Four extra-embryonic membranes form alongside the embryo. They surround it but are NOT part of the embryo's body — they are discarded at birth. They are common to all amniotes (reptiles, birds, mammals) but their relative importance differs.

I. YOLK SAC:
• Origin: Hypoblast lining the original blastocoel/yolk space.
• Wall: 2 layers — endoderm inside + visceral mesoderm outside.
• Functions:
– First haematopoietic site (makes the first blood cells of the embryo).
– Temporary early location and migration route of primordial germ cells (PGCs), which are specified from epiblast-derived progenitors — these later migrate to the genital ridges.
– In bird/reptile: stores yolk for nutrition.
– In mammals: small, regresses early — only a vestige remains attached to umbilical cord.
• Fate: regression; in mammals it leaves the vitelline duct remnant.

II. AMNION:
• Origin: From extra-embryonic somatopleure (epiblast + somatic mesoderm) folding over the embryo.
• Wall: 2 layers — ectoderm inside + somatic mesoderm outside.
• Function: The 'water bag' — encloses the embryo in amniotic fluid:
– Cushions against mechanical shock.
– Prevents dehydration.
– Allows free movement and growth.
– Permits limb/lung development by providing a fluid environment.
– Lubricates birth canal.
• At birth the amnion usually ruptures after the chorioallantois; its fluid contributes the later water bag.

III. ALLANTOIS:
• Origin: Outgrowth of the hindgut endoderm + visceral mesoderm.
• Wall: 2 layers — endoderm inside + splanchnic mesoderm outside.
• Function:
– In bird/reptile: stores fetal waste (uric acid) and provides gas exchange via shell.
– In mammal: expands rapidly + fuses with chorion → forms CHORIOALLANTOIC PLACENTA.
– Brings the embryo's blood vessels (umbilical aa. & v.) into close contact with chorion.
– Stores fetal urine (allantoic fluid) in mammals.
• At birth the chorioallantoic/allantochorionic sac generally ruptures first and releases the first water bag; the amniotic sac normally follows as the second water bag.

IV. CHORION:
• Origin: Outermost — formed by trophoblast + extra-embryonic somatic mesoderm.
• Lies in direct contact with maternal endometrium.
• Forms the FOETAL part of the placenta.
• Develops villi that may be diffuse, cotyledonary, zonary, or discoid (see next entry).
• Functions: nutrient + gas + waste exchange.
• In bird: lies just beneath shell; gas exchange with outside air.

V. UMBILICAL CORD (Mammal):
Twisted cord connecting placenta to fetal navel. Contains:
• 2 Umbilical Arteries (carry deoxygenated blood from foetus to placenta).
• 1 Umbilical Vein (carries oxygenated blood from placenta to foetus).
• Vestiges of vitelline duct + allantoic stalk.
• Wharton's jelly (mucous CT) cushions vessels.

VI. AT BIRTH:
• Chorioallantois/allantochorion usually ruptures first ('first water').
• Amnion usually ruptures later ('second water').
• Chorion delivered last with the placenta (= 'afterbirth').
• Retained placenta: In cattle, generally diagnosed when membranes remain about 24 hours after calving. Caused by milk fever, twinning, dystocia, infection.