File:Stage7 notochord.jpg

• Week 3, 15 - 17 days, 0.4 mm, embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side.
• Mesoderm lies between the ectoderm and endoderm as a continuous sheet except at the buccopharyngeal and cloacal membranes.
• These membranes have ectoderm and endoderm only and will lie at the rostral (head) and caudal (tail) of the gastrointestinal tract.
• From the primitive node (in the centre of the embryonic disc) a tube (shown in orange) extends under the ectoderm in the opposite direction to the primitive streak (shown in grey).
• This tube forms first the axial process then notochordal process, then finally the notochord.

Notochord

• The notochord is a key to embryonic folding and regulation of ectoderm and mesoderm differentiation.
• It lies in the rostrocordal axis
• the embryonic disc will fold either side of the notochord ventrally
  • pinching off a portion of the yolk sac to form the lining of the gastrointestinal tract.
• the embryonic disc will fold above and below the notochord ventrally
  • bringing the heart to upper abdomen
  • bringing the connecting stalk to the umbilicus

Stage 7 Mesoderm: axial | paraxial | intermediate | lateral plate

Features: embryonic disc, primitive node, primative streak, primative groove, yolk sac

Facts: Week 3, 15 - 17 days, 0.4 mm

View 1: embryonic disc, showing the epiblast viewed from the amniotic (dorsal) side.

Events: Gastrulation is continuing as cells migrate from the epiblast, continuing to form mesoderm.

• Blue dot (top) - buccopharyngeal membrane
• Blue dot (middle) - primitive node
• Blue dot (bottom) - cloacal membrane
• Orange line - axial mesoderm (future notochord)
• grey line - primitive streak

Stage 7 Links: :Carnegie stage 7 | Gastrulation | Mesoderm