Amnion, Umbilical cord, Yolk sac and Allantois


Made of amnioblasts and somatopleuric layer of extraembryonic mesoderm

The connecting stalk is only made of extraembryonic mesoderm

The amnion cavity obliterates extraembryonic coelom

Amniotic sac

Functions of amniotic fluid (made from amnioblasts and fetal urine):

  1. Cushions the baby
  2. Develops the suckling reflex
  3. Space for urine discharge
  4. Maintains constant temperature
  5. Antiseptic – cleanses vagina when the water breaks
  6. Allows movement of embryo – muscle development
  7. Bag of waters – dilates cervix gently


  1. Oligohydramnios – less amniotic fluid, adhesion of embryo to itsself and the amnion
  2. Polyhydramnios – more amniotic fluid, premature rupture

Umbilical Cord

Tubular sheath of amnion from placenta to umbilicus (naval)

Development of the cord:

1. Primitive umbilical ring – line of reflection between amnion and ectoderm

Image result for primitive umbilical ring

2. Primitive umbilical cord

  • Body stalk
  • Yolk sac
  • Part of allantois (later in the chapter)

3. Definitive umbilical cord

  • Wharton’s jelly (mucoid substance from extraembryonic mesoderm)
  • 2 umbilical arteries
  • 1 umbilical vein
  • Physiological hernia between 6th and 10th week


  • Short cord
  • Long cord – can wrap around fetus neck
  • Exomphalos – failure to reduce physiological umbilical hernia

  • Attachment to placenta can be eccentric, marginal or velamentous (surrounding fetal membranes)
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placenta attachments - eccentric, marginal, velamentous
  • One umbilical artery instead of two
  • False knots
umbilical cord knot
  • True knots – dangerous, obstruct blood flow
  • Two to three umbilical cords

Yolk Sac

1. Primary yolk sac – Heusers membrane

2. Secondary yolk sac – Extraembryonic coelom

3. Folding of embryonic disc – primitive gut and definitive yolk sac

The embryonic disc folds in a cranial-caudal and lateral direction, because the central area of the disc grows more then the periphery. This results in the incorporation of the yolk sac roof into the embryo forming the primitive gut which is divided into foregut, midgut and hindgut.

The part of the yolk sac which was not incorporated is known as definitive yolk sac and is connected to the primitive gut by vitellointestinal  duct at the midgut.

Vitelline vessels – network of vessels develop in splanchnopleuric mesoderm covering the secondary yolk sac

Functions of yolk sac:

  1. Roof forms primitive gut
  2. Caudal end forms allantois
  3. Gives primordial germ cells which migrate to developing gonads
  4. Some vitelline vessels form embryonic vessels


Allantoic vessels form the umbilical artery and vein

It’s a tubular invagination of the secondary yolk sac

It has 2 parts:

1. Intraembryonic part: forms  urachus which connects urinary bladder to umbilicus. After birth, the urachus is obliterated to form median umbilical ligament

2. Extraembryonic part: obliterated

Image result for allantois